Before using any image, you must make sure you're not violating someone else's rights. Just because a picture is posted at thousands of websites doesn't mean everyone is allowed to use it. Chances are some of those postings violate copyright. Do some in-depth research before using any image. If the image is listed as "public domain" you can use it without any problems. Other images may be used if you give credit to the person providing the image as stated in the website's TOS. Still others you must contact the photographer or artist and get their permission. Simply typing in the source is not enough. It takes time to figure out which you can use and which you can't, but in the long run, you'll be saving yourself from involvement in a law suit.