I printed the image I'm thinking of using and cut it out in a kind of decopague, kind of topographic image, which looks like this:
It's a shame I ran out of white paper, and had to use yellow, but at the same time you still get the contrast with the greys etc.
My 4 year old picked this design from 3 that i had drawn, he knew it was a tree truck with no hints from me at all, so it must look like a tree trunk. I'm quite happy with it overall, it has clarified a lot of possible problems in my head, and made the image come alive for me.
Now I just have to do the same thing with fabric!