Since recently I try to deliver presentation with more visual effects based on the Presentation Zen approach. Of course one of the problems is to find nice pictures.
I’m aware of the following sites:
If you select images from these sites make sure to check the usage license, some may require permission from the author, others may ask you to give credit to the author, etc. Also make sure to give the author credit for his work in any case. A common approach to give credit to the author is to add a slide to the presentation which lists the URL of the pictures.
I personally really liked stock.xchng; it made it easy to find beautiful pictures which have a very permissive licenses. My first impression from Flickr was not as amazing; there are simple to many pictures to select from. Peter Friese pointed out a program ViewFinder for the Mac but I have not yet tried that.
The major pay site with beautiful artwork is istockphoto. Really great images but you have to pay for them.
If you know other web sites for nice pictures please let me know. Or perhaps you want to share your tricks how to select good images at Flickr.