@KoiwiGal - The only thing is, if you're planning on selling bumper stickers then it's a good idea to make sure that your fonts and pictures are all free for commercial use.
Sometimes companies that customize bumper stickers will let you know if you shouldn't be using a particular image and sometimes they won't. But if you start selling them commercially and someone else owns the image it will often be found out and the person will have the right to sue you for damages.
If all you want is to get your idea onto some cheap bumper stickers then look for public domain images or sketch your own designs. As long as you can scan them in and convert them to a .jpg file you will be fine.