If, however, you are someone who feels that using these isn't a problem, have you noticed the fine print along the bottom that states the image you're looking at might be subject to copyright? Using them for free is theft, plain and simple. It's really only common sense to recognize that if you didn't create it, it's not yours to use without express permission from the artist or without paying for it.
The best option then, of course, is to make sure you purchase the right to use the images from a reliable online source, such as iCLIPART.com, Clipart.com, Acclaim Images and AnimationFactory.
The rules are different for every use. Regulations vary on the type of product you are creating with the image. Also, permissions will be unique to each site. For example, some might allow the images to be used commercially without editing, while another site's terms will state that they can only be used commercially if changes are made to them. Then there are that restrictions might be the norm rather than the exception. For example most sites strictly forbid use for print-on-demand and electronic distribution.
Should you go against the terms and get caught you are liable for damages that can be quite high. These companies will not be appeased by your argument that you attributed the image to them. Such action does not negate copyright infringement.
The bottom line is, if time is money, then it's well spent on reading and understanding the terms. if you're not sure, contact the company and ask before proceeding with a purchase.