Do cover images need to be original works, or require any kind of edits if you just pull a picture of the internet? A lot of them look like photoshops, at least.
No, not required, as far as I know. The original picture would probably need to be trimmed and cropped at least.
In a general legal sense, you shouldn't use copyrighted art unless it is under a licence that permits you to do so. However, CHYOA doesn't check the copyright status of every single image uploaded by users, and it's not something usually enforced by moderators.
As CHYOA is a site for written material, CHYOA can't judge whether non-written material might be subject to fair use or not. Also, see Copyright and DMCA Policy
