Make sure to read the image use guidelines or copyright policies on each site that you access.
Under certain stipulations images will often be available for use related to study or research:
Creative Commons licenses allow creators to get credit for their work as well as retain copyright while allowing others to copy, distribute, and make some use of their work (non-commercially). The creator is able to choose from a series of six licenses to find the one that best supports their work and intent.
Creative Commons content can be found on Google, Flickr and Wikiemdia Commons amongst other services.Simply look for a derivation of this symbol: