If an image covers the entire slide, where should I credit the author/licensor in a presentation?
Bottom right corner (maybe unobtrusive in gray, as not to interfere with the white background) seems right to me - you see this often in paper magazines, too. This way nobody can easily or accidentally strip the attribution. A [number] (even maybe just the slide number) and picture credits on the last slide would be the second-best choice.

And it's nice (for a change) to have someone respecting the license terms. Thank you :)