"On the cover" would be correct. In this situation, it's all about context: because there is an "inside" part of a book (the pages), the cover is considered to be the surface, so anything that appears there is "on." When we're looking only at a photo, the only "inside" part is what appears within the photo (since there are no pages bound to it).
I hope this helps answer your question. Please let me know if my answer is unclear or confusing. Thank you.