In addition to the asthetics, you'll want to consider the format the art is provided in. Raster, or pixelized images do not give as good of quality in print projects as vector, or eps images. If you want to change the colors to match your school colors, and if the images are going to appear large (on a banner or sign), be sure to look for vector eps images. You will need some type of vector based illustration program to change the colors. Coreldraw and Illustrator are the two most commonly used programs. If you don't have one of these programs, check with the Web site provider of the clipart. In many cases, they are glad to make color modifications on your behalf for a modest fee.
Before you purchase or use royalty-free clipart you should know the two main types of formats. Clipart can come in a vector format and a raster format.
Do make sure your cartoon advertising character reflects positive attributes: friendly, helpful, intelligent, likable and smart.