Raster format is also known as bitmap or BMP. They mean the same thing and are comprised of pixels, so when you enlarge them they become diluted. It's the most widely used clipart format and is usually all you need.
Another interesting place to look at for your image needs is Microsoft Office ClipArt in the Microsoft Office Program. As per the EULA of the Office Program, you can use these images in any document or for any work as long as you are not distributing only the image. Microsoft also allows access to its huge database of quality clipart and graphics at office.com. These images are of high resolution and some are even in vector format. You can download images and use them in your website or blog without a hassle. The Office.com Terms also mention that you can use these images in any document, even if it is for commercial purposes.
Independently owned restaurant owners such as pizza or Mexican restaurants need to be careful when they use clipart to promote their restaurant. Yes, it gives you an inexpensive solution, but it's easy to do more damage than good if you're not careful. After all, there is a reason why the big franchises pour so much money into their marketing.