I did this for my niece and nephew when I took them to WDW in May. I am not computer litterate in the least and it wasn't hard for me to do.
I cut and pasted the characters I wanted onto a word document. then I printed the pages out onto sticker paper. I bought the stuff that was a full sheet of sticker paper, not the one that is divided to use for address labels. You can find the sticker paper at Office Max and Staples type stores.
After I printed the pages out I cut them inot an oval shape. I left the white around the character because I stuck them onto 4X6" cards. You can do more intricate cutting if you want, but why bother?
I then put each of the 4X6 cards into it's own page of a little wallet size photo album. It came out really cute.
I did a search for US clipart and got some sites but I didn't open them. The descriptions appear to be pornography.
I then tried just the characters and Popeye was definately porn, but found some Spiderman art that you might be able to use. You might have to do some searches either on google or whatever search engin you like best. If I do find something I will let you know though.
Good luck with your hunting. Peggie