(I shouldn't have to make this disclaimer but these are for my own personal use! Not free to use!)
On Moderneopets, there's an option to have a CSS code to replace your Neopet's image with custom art, like in the early days! Here is my gallery of overlays I have made for my pets on the website.
If you like these and also have an account on the site, I'm open to making some for other people in exchange for items! My username is capybara #908!
Neomail if you want to buy this design for 450k NP, .PSD file included.