Applications :: Organize Images For Powerpoint/Keynote?

i give lectures at a local university and wonder if you could suggest a few programs as a repository/organizer for a bunch of images (most will come off the web). I realize that Iphoto can do some of this, however, its tendency to group images by date is not terribly useful to me. What I'd prefer is something like Papers that would let me preview and tag the images.

I wish I could be more specific; however, my imagination is hampered by my lack of familiarity with what software is out there.

Also, if you have any suggestions for organizing materials for presentations--I give 8-10 different 1-hour presentations per week-