Both image formats can produce excellent stamps; however simple images are always best in .eps and .wmf format to allow you to easily manipulate line thickness in the event the lines are too thin.
Various graphics and clip art are available on the Internet. Many of these items can be downloaded or printed for free. Downloading and printing images is a good way to find pictures or photographs of your child's favorite television character or band for a unique personalized touch. Print these graphics or clipart directly onto lignin-free and acid-free paper.
"Wikimedia Commons is a media file repository making available public domain and freely-licensed educational media content (images, sound and video clips) to everyone, in their own language. It acts as a common repository for the various projects of the Wikimedia Foundation, but you do not need to belong to one of those projects to use media hosted here. The repository is created and maintained not by paid archivists, but by volunteers."