Sorry, maybe I posted in a wrong place.
Anyway, I think I found the answer, or at least make it work now. My *.bpmn and *.ftl is OK, everything is the PNG file name in the JAR file.
My ProcessID: com.sample.bpmn.sampleInternalOrder
My PNG file:
com.sample.bpmn.sampleInternalOrder-image.png -> Don't work.
sampleInternalOrder-image.png -> Don't work.
Now I use two files:
com.sample.bpmn.sampleInternalOrder.png and sampleInternalOrder.png -> Work. So some of them make the diagram visible, I'll try the exactly one to work later.