A short story allows a lot of freedom in a response, even if you wrote in 'essay style' the marker might interpret that as being a deliberate part of your story. The marker could interpret your use of the formal language/structure of an essay as a deliberate decision i.e you aimed it toward an adult audience, and therefore used more sophisticated language.
Don't worry too much, if you related it to change and to the picture, I'm sure you'll be fine.