It was interesting.... so is the image suggestive then, or isn't it?
If we allow all cartoon porn because nobody has been victimized, then the above scenario is more simple - does the image originate from or use as its input an image requiring the suffering/exploitation of a non-consenting party?
This would cover fancy image-hiding methods because you would presumably run a script on some extant photo. You also wouldn't be able to paint your own copy of an abuse photo and claim artistry.
You could also say "all images of abuse, real or imagined", but Australia has shown that legislating thought-only is unproductive. It also wouldn't give much extra social benefit over a less restrictive real-only law.