Index: trunk/src/org/openstreetmap/josm/io/imagery/WMSImagery.java
===================================================================
--- trunk/src/org/openstreetmap/josm/io/imagery/WMSImagery.java	(revision 7171)
+++ trunk/src/org/openstreetmap/josm/io/imagery/WMSImagery.java	(revision 7175)
@@ -104,5 +104,6 @@
     public String buildGetMapUrl(Collection<LayerDetails> selectedLayers, String format) {
         return buildRootUrl()
-                + "FORMAT=" + format + "&VERSION=1.1.1&SERVICE=WMS&REQUEST=GetMap&LAYERS="
+                + "FORMAT=" + format + (imageFormatHasTransparency(format) ? "&TRANSPARENT=TRUE" : "")
+                + "&VERSION=1.1.1&SERVICE=WMS&REQUEST=GetMap&LAYERS="
                 + Utils.join(",", Utils.transform(selectedLayers, new Utils.Function<LayerDetails, String>() {
             @Override
@@ -153,4 +154,6 @@
         }
         String incomingData = ba.toString();
+        Main.debug("Server response to Capabilities request:");
+        Main.debug(incomingData);
 
         try {
@@ -221,4 +224,9 @@
                 || format.startsWith("image/bmp") && ImageIO.getImageReadersBySuffix("bmp").hasNext();
     }
+    
+    static boolean imageFormatHasTransparency(final String format) {
+        return format.startsWith("image/png") || format.startsWith("image/gif") 
+                || format.startsWith("image/svg") || format.startsWith("image/tiff");
+    }
 
     public ImageryInfo toImageryInfo(String name, Collection<LayerDetails> selectedLayers) {