Didn't your parents teach you to ask for permission? Some graphic collections are sold for publication, but often they are expensive. If you use a graphic that you find online, or that came in a collection, you must receive permission to use the graphic. If you do not have permission to use an image, you will be violating the graphic designer's copyright.
*Never simply assume the graphic is "ok," you need to make absolutely sure or you open yourself and your book up to lawsuits and copyright infringement violations.
*There are plenty of sites available that offer art for cover or text images at very low costs or even free of charge. Look around.