There are several ways you could show the marker that you are actually using the stimulus and one of them is actually just using it as a central element to your text in terms of a plot device or setting.
IF you do decide to use the stimulus for what is presented to you, obviously if you want to be able to grade well among other creative stories make sure that you incorporate it with a purpose and not as simple as saying the stimulus multiple times throughout your story.
In order to do this make sure that your actual story targets something to do with discovery, so let's say it's a picture of a book you could say that your main character made a discovery through the book, i.e. found out something to do with his long lost brother...
Honestly, I'd say your question is sort of redundant as you have to use the stimulus and to say using it 'literally' or not is confusing. Good stimulus incorporation involves using the stimulus for what it stimulates in terms of discovery... if you mean how explicit you want to use the stimulus I would say do it to your hearts desire as long as it works.