If you haven’t noticed it yet, Google Image Search allows you to filter by license. This means that you can search for only images that can be reused. These images include GNU Free Documentation, Creative Common, or public domain images.
Go to the Advanced Google Image Search and notice the drop-down labeled: Usage Rights
Select the license that you would like to filter for and search away! The quality of images is not great but at least you know you’re acting within legal limits. If you’re looking for a good deal on high quality images try Visco Images.
For more information see the Google Documentation on Image Usage Rights.