diff --git a/manual/Tasks/imageio.html b/manual/Tasks/imageio.html
index cce503ca9..65d6815bd 100644
--- a/manual/Tasks/imageio.html
+++ b/manual/Tasks/imageio.html
@@ -29,7 +29,12 @@
 <p>Applies a chain of image operations on a set of files.</p>
 <p>Uses AWT and ImageIO; replaces <a href="image.html">image</a> task for Java 9+. The task can be
 used with Java 8 as well, see parameter table for limitations.</p>
-
+<p><strong>Note</strong>: this task tries to stay as close as possible to syntax and semantics
+of <code>image</code> task. However, it uses <var>format</var> attribute rather
+than <var>encoding</var> attribute, because the latter is a misnomer: almost all tasks use similar
+attributes for character encodings in files, file names or other strings. Also,
+when <var>format</var> is not specified, its value is defined by the format of the first processed
+image file.</p>
 <h5>Overview of used datatypes</h5>
 <svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" fill-opacity="1"
      color-rendering="auto" color-interpolation="auto" text-rendering="auto" stroke="black"
@@ -390,12 +395,12 @@ used with Java 8 as well, see parameter table for limitations.</p>
     <td>Yes, unless nested fileset is used</td>
   </tr>
   <tr>
-    <td>encoding</td>
+    <td>format</td>
     <td>Image format.<br/>Valid (case insensitive)
       are: <q>bmp</q>, <q>gif</q>, <q>jpeg</q>, <q>jpg</q>, <q>png</q>, <q>tif</q>, <q>tiff</q>, <q>wbmp</q>
       (Java 8 VM lacks support for TIFF, which can be provided
       by <a href="../install.html#librarydependencies">external libraries</a>).</td>
-    <td>No; defaults to the format of the first original file</td>
+    <td>No; defaults to the format of the (first) original file</td>
   </tr>
   <tr>
     <td>overwrite</td>
diff --git a/src/main/org/apache/tools/ant/taskdefs/optional/image/ImageIOTask.java b/src/main/org/apache/tools/ant/taskdefs/optional/image/ImageIOTask.java
index 5e4289c1e..ce25c3898 100644
--- a/src/main/org/apache/tools/ant/taskdefs/optional/image/ImageIOTask.java
+++ b/src/main/org/apache/tools/ant/taskdefs/optional/image/ImageIOTask.java
@@ -20,6 +20,7 @@ package org.apache.tools.ant.taskdefs.optional.image;
 import org.apache.tools.ant.BuildException;
 import org.apache.tools.ant.Project;
 import org.apache.tools.ant.taskdefs.MatchingTask;
+import org.apache.tools.ant.types.EnumeratedAttribute;
 import org.apache.tools.ant.types.FileSet;
 import org.apache.tools.ant.types.Mapper;
 import org.apache.tools.ant.types.optional.imageio.Draw;
@@ -59,7 +60,7 @@ public class ImageIOTask extends MatchingTask {
     private File srcDir = null;
     private File destDir = null;
 
-    private String format;
+    private ImageFormat outputFormat;
 
     private boolean garbageCollect = false;
 
@@ -93,11 +94,11 @@ public class ImageIOTask extends MatchingTask {
     }
 
     /**
-     * Set the image format.
-     * @param encoding the String image format.
+     * Set the output image format.
+     * @param format an ImageFormat.
      */
-    public void setEncoding(String encoding) {
-        format = encoding;
+    public void setFormat(ImageFormat format) {
+        outputFormat = format;
     }
 
     /**
@@ -281,8 +282,8 @@ public class ImageIOTask extends MatchingTask {
                 return;
             }
             ImageReader reader = readers.next();
-            if (format == null) {
-                format = reader.getFormatName();
+            if (outputFormat == null) {
+                outputFormat = new ImageFormat(reader.getFormatName());
             }
             reader.setInput(input);
 
@@ -308,7 +309,7 @@ public class ImageIOTask extends MatchingTask {
                 newFile.delete();
             }
 
-            if (!ImageIO.write(image, format, newFile)) {
+            if (!ImageIO.write(image, outputFormat.getValue(), newFile)) {
                 log("Failed to save the transformed file");
             }
         } catch (IOException | RuntimeException err) {
@@ -378,11 +379,33 @@ public class ImageIOTask extends MatchingTask {
         if (srcDir == null && destDir == null) {
             throw new BuildException("Specify the destDir, or the srcDir.");
         }
-        if (format != null && !Arrays.asList(ImageIO.getReaderFormatNames()).contains(format)) {
-            throw new BuildException("Unknown image format '" + format + "'"
-                + System.lineSeparator() + "Use any of "
-                    + Arrays.stream(ImageIO.getReaderFormatNames()).sorted()
-                    .collect(Collectors.joining(", ")));
+    }
+
+    /**
+     * defines acceptable image formats.
+     */
+    public static class ImageFormat extends EnumeratedAttribute {
+
+        private static final String[] VALUES = ImageIO.getReaderFormatNames();
+
+        /**
+         * Constructor
+         */
+        public ImageFormat() {
+        }
+
+        /**
+         * Constructor using a string.
+         * @param value the value of the attribute
+         */
+        public ImageFormat(String value) {
+            setValue(value);
+        }
+
+        /** {@inheritDoc}. */
+        @Override
+        public String[] getValues() {
+            return VALUES;
         }
     }
 }