It is sometimes thought that materials available on the open web are in the public domain, or otherwise free to reuse. In fact, most works posted on the web are protected by copyright, and therefore reproducing or distributing web content may require permission from the copyright holder, just as with print materials. However, simply creating links to legally posted web materials typically does not require permission.
There are many websites that have Creative Commons or public domain images that can be use with no permissions or fees. A simple Google search can locate them.
A copyright exception that could be useful here -- the "useful article" provision at 17 USC 113(c) which states:In the case of a work lawfully reproduced in useful articles that have been offered for sale or other distribution to the public, copyright does not include any right to prevent the making, distribution, or display of pictures or photographs of such articles in connection with advertisements or commentaries related to the distribution or display of such articles, or in connection with news reports.