Fortunately I see more and more people who abandon their boring bullet-point overloaded slides and switch over to more visual slides. However once you make the decision to use images for illustrating your thoughts you might ask yourself where you can find them. My favorite source for images by far is Flickr, an online photo site.

A lot of photographers on Flickr release their pictures under the Creative Commons license thereby giving you the right to use these pictures under certain conditions e.g. the author usually asks you to clearly state who generated the content.

In order to find pictures which are covered by the CC license make sure to use Flickr's advanced search. See the following screenshot.

The following slides are from a presentation I gave on unit testing and use pictures from Flickr. They should give you a good idea of the image quality you can get and what you can do with them.

For more ideas regarding stock images hop over to presentation zen.