Just find the pic's you want, resize them to thumbnails or similar; go to your user options and select signature, copy and paste the img code of each one but without any breaks so each pic' code joins the next and it should work! If you try it with two pic's first, you'll get the hang of it without wasting time and can go back to add any others.
You don't need to download anything as long as you have something like photobucket.