Just some thoughts:
1) Wrong assersion messages:
testEmpty -> "image withOUT resource reference should be statefull"
testResource -> "image with resource should be statefull"
2) Confusing javadoc in LocalizedImageResource#isStateless(). It says:
@return If it is stateless (if resource is null)
But current implementation checks if resource is NOT null.
3) I still doesn't understand why rr-less image is statefull. What's it's state? It doesn't have any variables defined -> no state. And image with rr has state - it contains knowledge about what exact resource this image references.
Look at your test case. Empty images has state, but non-empty image doesn't have state. This is very confusing.
So I strongly recommend you to take a deep breath and thing about this issue once more.