If you use designer in guvnor you just need to press png button on the bottom left of the screen. When using eclipse to design your process there is a button in the toolbar, on the top right i guess that has a rectangle shape and give the file name as {process-id}-image.png (not sure about this one something like DefaultPackage.testProcess-image.png) and upload it to guvnor.
as for the forms. when using online designer there is a button on top right that auto creates forms. or you can write your own freemarker template with name {taskName}-taskform.ftl.
Hope this helps.
Okan