I'm going to change my roommates computer background when she leaves her laptop open
Is this a prank she is going to be happy with?
Of course not, I'll get in trouble
It may be a harmless prank, but you still shouldn't mess with her personal stuff. Sometimes people forget the source of their wallpaper, and it will be deleted forever if you change it.
I have to have my fun somehow.
There are plenty of other ways to have fun with them. See what makes then angry and what doesn't, then find an appropriate practical joke to play with them. Remember, if they can't laugh with you, then the joke is going too far.