Image captions are not currently discussed in the Manual of Style (for Undertale or Deltarune), although I think they should be.
In particular, I think it would be good to have clarification on how image captions should be written. Typically on a wiki, image captions follow standard grammatical rules — they end in a period only if they form a complete sentence. (For example, see Wikipedia’s policy.) While image captions can be complete sentences, more commonly they are sentence fragments (typically just noun phrases).
However, on the Undertale and Deltarune wikis, it seems to be fairly common to place periods after every single image caption, regardless of whether it is a sentence or a noun phrase. Is this something that there is a consensus to make an official policy? Or would following standard grammatical rules be preferred? Either way, I think this is something that should be explicitly discussed in the Manuals of Style.