If you use Google Images to download your images and clip art, do you check to see if the images are copyrighted? Some owners charge for using their images and some just want you to acknowledge them by citing the source where you downloaded the image or clip art. Before you use any image or clip art on the Internet, it is your responsibility to make sure you use them without infringing on someone's intellectual property. See the video on the left on how you can find free-to-use images.
Searching for images can be tiresome. Many sites are sites with links to images and others want to sell images. Some sites offer royalty free images. Royalty free refers to the right to use copyrighted material or intellectual property without the need to pay royalties or license fees for each use or per volume sold, or some time period of use or sales. (Reference: http://en.wikipedia.org/wiki/Royalty-free) So be careful!
Here are some sites that offer free clip art and images. However, be sure to check each site to see if the owner of the image or clip art would like a citation to the source.