I'm correcting my 7th graders' writing assignment in which they're given three pictures of a fat boy who decides to go on a diet and finally becomes thinner. I've been told that using present simple can create an effect of "bringing the character to the present time" and therefore sounds more lively to the reader. But I'm also confused because the pictures certainly deal with something already happened to that boy, so simple past sounds more logical to me. And what if a students starts a sentence like "One day...."? I don't think simple present can be applied here.
Consider that photo captions use the present tense almost exclusively, no matter how old the picture is:
RMS Titanic leaves the docks at Southampton on her maiden voyage to New York in this 1912 photo.
It's not hard to see why this is: when you look at a photograph, you are in a sense transported to the moment in time at which it was taken, so it's natural to use the present tense to describe it.
Similarly, your 7th graders are providing narration for a series of pictures (you didn't say whether they are photos or drawings, but it doesn't really matter), and the reader is expected to read the narration at the same time that he or she views the pictures. I would want to see present tense.