istockphoto.com

iStockphoto offers a ton of professional pictures, illustrations and audio clips for business. This can be used for a ton of different uses in your business.

GraphicsFactory.com

Graphics Factory is has unlimited downloads of pictures, clip art, backgrounds, and words for only $49.95/year with unlimited downloads.

This is more of cartoons and clip art, but it evolves continuously and is an inexpensive way to have access to a ton of images.