You are allowed to use a custom image representing your pet to overlay on the original pet image as long as the pet&;s name, true color, specie, gender, and owner are still visible and not misrepresented in any way. Misrepresenting any details -- including stats -- is not allowed. In short, overlays are okay as long as textual details are not covered, and current, unaltered official art is not used. These same rules apply to minion and treasure chest overlays as well.