The sites in this list will help you find iamges for your online course. Most have images whose creators have offered their material for use under a Creative Commons license.
If you’re not familiar with Creative Commons, here is their website with good explanation.
Flickr Storm - While this site pretty much does the same as Flickr’s own search tool, Flickr Storm allows the user to set up a search and then save them for later reuse. The advanced search also allows searching for Creative Commons licensed pictures.
yotophoto - Use this site to search the web (including Flickr) for photographs licensed under Creative Commons and other open license systems (copyleft)
Wikipedia free image resources - This section of the online encyclopedia Wikipedia offers a short list of links to pages all over the world that offer royalty-free and Creative Commons licensed pictures in specific subject areas.
National Science Foundation multimedia gallery - The US government has a large collection of images and other media that can be used for almost any non commercial purpose. This site offers a search tool to find them, most of which are oriented around science topics.
Image Base - This collection of stock photography by professional photographers is licensed for anyone to use for non-commercial purposes. This material is more sophisticated and general than some other sites in this list
PD Photo - Another large collection of public domain, royalty free stock photographs.