We’ve all been tempted to use images found on Google & Yahoo image searches. Unfortunately, “borrowing” these images will almost certainly bring you copyright troubles. So, I turn to two of my favorite sources for images:
Open Clip Art Library is a great source of illustrations that can be freely used. All the files are available as PNG or SVG filetypes.
Dreamstime has a huge collection of royalty free images at very reasonable prices. I really like their wide selection of photography.