The small manifest
pet.json tells the client the pet identifier, display name, description and image filename. Keep it beside spritesheet.webp in the same pet folder. Renaming the image without updating the manifest breaks that reference.
CodePets
What is pet.json?
Keep the original pair
Our current downloads use the original nine-row atlas format. Do not mark a nine-row image as spriteVersionNumber 2: that format requires additional directional artwork. Keep an untouched backup before editing.