Well, for example, it could be relevant for comics, for images with stories as text in the image, or stories in the description, or to refer to the author of a fanfic (not included) with which the image is associated (as a cover or an illustration, etc.)
@Background Pony #C170
Assuming you’re right (about which we cannot be sure), there’s still the question of the difference between “author” and “writer”.
I honestly didn’t even notice they existed. They seem to be used either to identify the author of a fanfic the the image is for or based on, or on some really old text posts, to identify the person who wrote the text. Either way, they aren’t a part of the site’s copyright processes, so they don’t have any ‘official’ use here. Having both doesn’t seem to seem to be a problem, though.
The difference is that an author is the person who comes up with and directs the substantial idea, plot, and content of a story.
A writer is anyone who writes the text. They aren’t always the same as an author because they might be writing the specific text but under the direction of someone else who’s actually coming up with the ideas driving it.
Don’t know if this matters in practicality, since I don’t think it comes up much as a distinction in most pictures of characters writing.
I think normally “author” should be used as the prefix. “Writer” is more for works with multiple authors broken up by role, to distinguish the role, to my understanding.