diff --git a/src/main/java/lc/captchas/FontFunCaptcha.java b/src/main/java/lc/captchas/FontFunCaptcha.java
index 1007078..8dbea52 100644
--- a/src/main/java/lc/captchas/FontFunCaptcha.java
+++ b/src/main/java/lc/captchas/FontFunCaptcha.java
@@ -1,6 +1,5 @@
 package lc.captchas;
 
-import javax.imageio.ImageIO;
 import java.awt.*;
 import java.awt.image.BufferedImage;
 import java.io.ByteArrayOutputStream;
@@ -10,6 +9,7 @@ import java.util.Map;
 import java.util.List;
 import lc.captchas.interfaces.Challenge;
 import lc.captchas.interfaces.ChallengeProvider;
+import lc.misc.PngImageWriter;
 import lc.misc.HelperFunctions;
 
 public class FontFunCaptcha implements ChallengeProvider {
@@ -74,7 +74,7 @@ public class FontFunCaptcha implements ChallengeProvider {
     graphics2D.dispose();
     ByteArrayOutputStream baos = new ByteArrayOutputStream();
     try {
-      ImageIO.write(img, "png", baos);
+      PngImageWriter.write(baos, img);
     } catch (Exception e) {
       e.printStackTrace();
     }
diff --git a/src/main/java/lc/captchas/ShadowTextCaptcha.java b/src/main/java/lc/captchas/ShadowTextCaptcha.java
index b510b9a..a797c50 100644
--- a/src/main/java/lc/captchas/ShadowTextCaptcha.java
+++ b/src/main/java/lc/captchas/ShadowTextCaptcha.java
@@ -1,6 +1,5 @@
 package lc.captchas;
 
-import javax.imageio.ImageIO;
 import java.awt.Graphics2D;
 import java.awt.RenderingHints;
 import java.awt.Color;
@@ -14,6 +13,7 @@ import java.util.Map;
 import java.util.List;
 
 import lc.misc.HelperFunctions;
+import lc.misc.PngImageWriter;
 import lc.captchas.interfaces.Challenge;
 import lc.captchas.interfaces.ChallengeProvider;
 
@@ -67,7 +67,7 @@ public class ShadowTextCaptcha implements ChallengeProvider {
     g2d.dispose();
     ByteArrayOutputStream baos = new ByteArrayOutputStream();
     try {
-      ImageIO.write(img2, "png", baos);
+      PngImageWriter.write(baos, img2);
     } catch (Exception e) {
       e.printStackTrace();
     }
diff --git a/src/main/java/lc/misc/HelperFunctions.java b/src/main/java/lc/misc/HelperFunctions.java
index 4e77821..22b3a8a 100644
--- a/src/main/java/lc/misc/HelperFunctions.java
+++ b/src/main/java/lc/misc/HelperFunctions.java
@@ -7,9 +7,9 @@ public class HelperFunctions {
 
   private static Random random = new Random();
 
-  synchronized public static void setSeed(long seed){
+  public static synchronized void setSeed(long seed) {
     random.setSeed(seed);
-  } 
+  }
 
   public static void setRenderingHints(Graphics2D g2d) {
     g2d.setRenderingHint(
@@ -38,12 +38,11 @@ public class HelperFunctions {
     return stringBuilder.toString();
   }
 
-  synchronized public static int randomNumber(int min, int max) {
+  public static synchronized int randomNumber(int min, int max) {
     return random.nextInt((max - min) + 1) + min;
   }
 
-  synchronized public static int randomNumber(int bound) {
-   return random.nextInt(bound);
+  public static synchronized int randomNumber(int bound) {
+    return random.nextInt(bound);
   }
-  
 }
diff --git a/src/main/java/lc/misc/PngImageWriter.java b/src/main/java/lc/misc/PngImageWriter.java
new file mode 100644
index 0000000..e13b783
--- /dev/null
+++ b/src/main/java/lc/misc/PngImageWriter.java
@@ -0,0 +1,68 @@
+package lc.misc;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.util.Iterator;
+
+import javax.imageio.IIOImage;
+import javax.imageio.ImageIO;
+import javax.imageio.ImageTypeSpecifier;
+import javax.imageio.ImageWriteParam;
+import javax.imageio.ImageWriter;
+import javax.imageio.metadata.IIOInvalidTreeException;
+import javax.imageio.metadata.IIOMetadata;
+import javax.imageio.metadata.IIOMetadataNode;
+import javax.imageio.stream.ImageOutputStream;
+import java.awt.image.BufferedImage;
+
+public class PngImageWriter {
+
+  static final int DPI = 245;
+  static final double INCH_2_CM = 2.54;
+
+  public static void write(ByteArrayOutputStream boas, BufferedImage gridImage) throws IOException {
+    final String formatName = "png";
+    for (Iterator<ImageWriter> iw = ImageIO.getImageWritersByFormatName(formatName);
+        iw.hasNext(); ) {
+      ImageWriter writer = iw.next();
+      ImageWriteParam writeParam = writer.getDefaultWriteParam();
+      ImageTypeSpecifier typeSpecifier = ImageTypeSpecifier.createFromBufferedImageType(BufferedImage.TYPE_INT_RGB);
+      IIOMetadata metadata = writer.getDefaultImageMetadata(typeSpecifier, writeParam);
+      if (metadata.isReadOnly() || !metadata.isStandardMetadataFormatSupported()) {
+        continue;
+      }
+
+      setDPIMeta(metadata);
+
+      final ImageOutputStream stream = ImageIO.createImageOutputStream(boas);
+      try {
+        writer.setOutput(stream);
+        writer.write(metadata, new IIOImage(gridImage, null, metadata), writeParam);
+      } finally {
+        stream.close();
+      }
+      break;
+    }
+  }
+
+  private static void setDPIMeta(IIOMetadata metadata) throws IIOInvalidTreeException {
+
+    // for PNG, it's dots per millimeter
+    double dotsPerMilli = 1.0 * DPI / 10 / INCH_2_CM;
+
+    IIOMetadataNode horiz = new IIOMetadataNode("HorizontalPixelSize");
+    horiz.setAttribute("value", Double.toString(dotsPerMilli));
+
+    IIOMetadataNode vert = new IIOMetadataNode("VerticalPixelSize");
+    vert.setAttribute("value", Double.toString(dotsPerMilli));
+
+    IIOMetadataNode dim = new IIOMetadataNode("Dimension");
+    dim.appendChild(horiz);
+    dim.appendChild(vert);
+
+    IIOMetadataNode root = new IIOMetadataNode("javax_imageio_1.0");
+    root.appendChild(dim);
+
+    metadata.mergeTree("javax_imageio_1.0", root);
+  }
+}
diff --git a/src/main/scala/lc/captchas/DebugCaptcha.scala b/src/main/scala/lc/captchas/DebugCaptcha.scala
index 787b6cf..f0add52 100644
--- a/src/main/scala/lc/captchas/DebugCaptcha.scala
+++ b/src/main/scala/lc/captchas/DebugCaptcha.scala
@@ -1,6 +1,5 @@
 package lc.captchas
 
-import javax.imageio.ImageIO
 import java.awt.Color
 import java.awt.Font
 import java.awt.font.TextLayout
@@ -12,6 +11,7 @@ import java.util.List
 import lc.misc.HelperFunctions
 import lc.captchas.interfaces.Challenge
 import lc.captchas.interfaces.ChallengeProvider
+import lc.misc.PngImageWriter
 
 /** This captcha is only for debugging purposes. It creates very simple captchas that are deliberately easy to solve with OCR engines */
 class DebugCaptcha extends ChallengeProvider {
@@ -53,7 +53,7 @@ class DebugCaptcha extends ChallengeProvider {
     graphics2D.dispose()
     val baos = new ByteArrayOutputStream()
     try {
-      ImageIO.write(img, "png", baos)
+      PngImageWriter.write(baos, img);
     } catch {
       case e: Exception =>
         e.printStackTrace()
diff --git a/src/main/scala/lc/captchas/FilterChallenge.scala b/src/main/scala/lc/captchas/FilterChallenge.scala
index 8c21e78..c7f2b70 100644
--- a/src/main/scala/lc/captchas/FilterChallenge.scala
+++ b/src/main/scala/lc/captchas/FilterChallenge.scala
@@ -8,6 +8,8 @@ import java.awt.Color
 import lc.captchas.interfaces.ChallengeProvider
 import lc.captchas.interfaces.Challenge
 import java.util.{List => JavaList, Map => JavaMap}
+import java.io.ByteArrayOutputStream
+import lc.misc.PngImageWriter
 
 class FilterChallenge extends ChallengeProvider {
   def getId = "FilterChallenge"
@@ -41,7 +43,15 @@ class FilterChallenge extends ChallengeProvider {
     var image = ImmutableImage.fromAwt(canvas)
     val s = scala.util.Random.nextInt(2)
     image = filterTypes(s).applyFilter(image)
-    new Challenge(image.bytes(new nio.PngWriter()), "image/png", secret)
+    val img = image.awt()
+    val baos = new ByteArrayOutputStream()
+    try {
+      PngImageWriter.write(baos, img);
+    } catch {
+      case e: Exception =>
+        e.printStackTrace()
+    }
+    new Challenge(baos.toByteArray, "image/png", secret)
   }
   def checkAnswer(secret: String, answer: String): Boolean = {
     secret == answer
diff --git a/src/main/scala/lc/captchas/LabelCaptcha.scala b/src/main/scala/lc/captchas/LabelCaptcha.scala
index cd008e7..18048f8 100644
--- a/src/main/scala/lc/captchas/LabelCaptcha.scala
+++ b/src/main/scala/lc/captchas/LabelCaptcha.scala
@@ -10,6 +10,7 @@ import java.awt.Color
 import lc.captchas.interfaces.ChallengeProvider
 import lc.captchas.interfaces.Challenge
 import java.util.{List => JavaList, Map => JavaMap}
+import lc.misc.PngImageWriter
 
 class LabelCaptcha extends ChallengeProvider {
   private var knownFiles = new File("known").list.toList
@@ -49,7 +50,7 @@ class LabelCaptcha extends ChallengeProvider {
 
       val token = encrypt(knownImageFile + "," + unknownImageFile)
       val baos = new ByteArrayOutputStream()
-      ImageIO.write(mergedImage, "png", baos)
+      PngImageWriter.write(baos, mergedImage);
 
       new Challenge(baos.toByteArray(), "image/png", token)
     }