I've never used wallpaper on my A1200 as it eats up chipram and free pen screen colours. I much prefer using a background copper effect like CopperDemon:

http://classicwb.abime.net/classicweb/advsp.htm

If you want to use some, make sure you design it to be as small as possible with a limited (less than your screenmode) palette so it doesn't dither and look horrible. I've seen screenshots of people in the past who've made really good but small background pictures. Here is a great example:

http://eab.abime.net/showpost.php?p=...&postcount=187

If you haven't already looked, Aminet may have some:

http://aminet.net/