the markers would have to be pretty dumb if they need to be directly told what picture you used. If you included one of the stimulus in your creative writing. You're set
I doubt they're going to take marks if you haven't written what stimulus u used. The supervisor never said to write it down, and it doesn't say anywhere in the booklet that u have to
I wouldn't think it matters. I did write it on the front of the booklet, but as long as the reader can identify which picture you are basing your creative story on, it should be fine. They are experienced teachers, they should identify quickly what you are writing about.
The only difference is that, say if the marker knows which stimulus your using, he/she will instantly know how well your are using it, instead of making a judgement when he/she works it out.