Adding dispose() as suggested by Jürg Lehni on helma-dev.

src/helma/image/ImageWrapper.java

@@ -81,6 +81,16 @@ public abstract class ImageWrapper {
     }
 
 
+    /**
+     * Dispose the Graphics context and null out the image.
+     */
+    public void dispose() {
+        if (img != null) {
+            g.dispose();
+            img = null;
+        }
+    }
+
     /**
      *  Set the font used to write on this image.
      */