I came accross a site today: http://www.aboutpixel.de/
This site features free (as in beer), high resolution pictures. The only thing you have to do to be able to use the pictures in your presentations is mentioning the site and the photographer/artist. This can be done right below the picture or in the imprint.
So, if you happen to give a presentation in one of the lectures I currently attend: give me something to look at :)