Images such as artwork, photographs, drawings, etc. are protected by copyright laws, even if you can find them online. When using images, make sure that they are either in the public domain or that your use of the image falls under fair use.
Tips for Using Images
Use low resolution or thumbnail versions when possible
Place the image in a new context or use it for a new purpose
Use only the part of the image that is needed
Use photographs of people in public settings or taken from a public location
Avoid images of well-known figures
Avoid photographs that invade privacy
Free Images
The following sites have images that are in the public domain and free to use. However, this does not mean you are allowed to publish or post these images yourself.