It's actually quite easy. Adding custom sprites or custom costumes to unique characters goes like this:

You extract the game's resources folders, (resource includes rebirth, afterbirth and afterbirth+ stuff) and resources-dlc3 includes repentance stuff. Once done, you head over to gfx, characters, costumes and then search for your desired custome by file name. If it ends with a prefix such as "blue, black, white" then delete it off the filename.

Once done that, edit your desired outfit and then put it on the character's respective folder. So for BB, you'd go: *modname*/resources or resources-dlc3/gfx/characters/costumes_bluebaby or with characters that don't share the same costumes folder, you'd go *charactername* with a b at the end.

And done.

Every character has slots for custom outfits, as long as the original item or transformation exists as a costume in the base game then you an add a custom one to anyone.

Adding new costumes to items that don't have one is more complicated, you'd have to dwelve into lua stuff, which I am not familiarized with.

If you don't really understand my instructions, you can go to your game's folder and take a look at my mod's file structure. Following it should make things easier to understand.

thank you