I always try to use Wikimedia Commons (http://commons.wikimedia.org) if I don't want to clutter a poster or presentation with citations. Hope this helps!

Here at UC Davis, we have found that a very good source for images for many of the researchers and scientists we work with is Shutterstock.com. We purchase a yearly contract with them for use rights with no citation required. Typically we would have you (Mandy) the client go on and search an image you would like to use and we download it for use on your presentation,materials, poster etc. However, we work with the client to create and produce the high end product that will get noticed and you applause. We would not simply send the image to the client, but provide it as part of our service. Cost per image to the client is $8.00. So in your case, you might consider setting up a contract with Shutterstock through your department for all of your colleagues to use and share the cost. I Hope this helps.

If your use is educational & non-commercial, then most material in the public domain can ce used with a standard source citation (which you can add under an image in small print). I have also found that many photographers/artists will give free permission for this type of work. The copyright law of most countries allows this type of usage.

For commercial use or work where you do not wish to cite permissions and sources, I would go down the route above .... or use your own work! I have created my own scientific drawings for use in my published work & also taken photos that I have manipulated on the computer for such use.

Be creative but above all, be honest. Finding & using images for non-commercial work is nowadays an uncomplicated process.