Making playpen backgrounds is pretty easy. You can make tiled wallpapers or full-sized images so the playpen will look almost like a playscene (especially for Petz 2 and Petz 3 where there is no editor and/or no playscene capability). This guide is for making tiled backgrounds that repeat to fill the playpen. Below are the steps I use to create them.
Creating The Tile: I normally start off with a 100x100 pixel image, but you can start off with whatever size you want. Images must be 8-bit, 256 colors (which are specific). I made a template that uses those colors. You can get the palette if you open one of the wallpapers that come with the games. (I use PSP 7 and when I open one of the files the palette is available automatically).
Decorating The Tile: Now this is the fun part. Anything goes. I use dingbats, the airbrush tool, and other tools available. Be creative and let your imagination run wild.
Lining Up The Tile: Once you have your tile created, you will want to make sure when you fill the playpen, that it all lines up nicely, or it might look like a big old mess. What you want is to make your tile into a seamless image, so when it tiles in the playpen, you won't see where one tile begins and the other ends. Here is an old tutorial on wayback that will help. Most modern graphics programs can probably do this as well. Gimp is the program I use to make seamless tiles.
Saving The Tile: Once you are satisfied with your tile, save the image. It must be an 8-bit 256 color BMP file. If you don't use the petz color palette some colors will look different. The wallpaper goes in the /Resource/Wallpaper folder of your Petz game.