It's possible, but you'll also have to create a Java version of your custom node types If you don't mind that, you could add the compiled .class files to JDiagram.jar, and call Diagram.registerItemClass in an override of Applet.init(), using the same class ID as on the server side. Now when the applet starts, it will load the diagram from an XML string passed from the server, and will create instances of the Java classes registered for the class id saved as attribute of the <node> elements.

If all you need is a shape that displays an image with a label below, you could do that using ordinary ShapeNodes by setting the following properties for them: Transparent = true, ImageAlign = Top, TextFormat = new StringFormat(Center, Far). Now if the shape height is set to the sum of the image and text heights, it should look the same as the icon nodes from the sample.

I hope that helps,
Stoyan