Well it depends what you're using to make the images. :o I found a thread on Mysidias sister project site Myadopts, its kind of the easier way to make a simple adoptable site since it sets it all up, but you cant access all the programming stuff, move the site, and the servers not too good since it has a million sites on it. o.o' Anyway, its this thread:
http://www.myadopts.com/forum/showthread.php?tid=7584 Or if you use photoshop its easy, you just click on the little eye on the background layer and it goes away. ^_^