I couldn't find anything about rules surrounding overlay images in the rules center, not sure if there's a thread somewhere else about it but I couldn't find it... basically I have this Golden Wyllop named Plinkachoo which I think is super cute but I actually hate rats lol so I want him to look more like Pikachu instead of a Wyllop but I didn't know if I could just make art of Pikachu and use that as an overlay since it's a copyrighted character? Also how close to the original pet do you have to stay? Thanks!

the rules are listed on this page. it is admittedly a bit unintuitive as you have to actually click the 'upload' button to get there, which most people wouldn't do before they have the image ready, and you can't know if your image is ready until you've read the rules, but you can't read the rules until you've clicked the button... yeah it's not ideal.

at the bottom of the page, in small text, it says:

awesome thanks!! That's a good idea actually!