The days of cartoon-ish clip art are over. The trend in slide presentations is photos. That is, high quality, legal, royalty-free photos. You can find them by searching the web using these key phrases: creative commons, free photos, wikimedia commons.
Here are a few sources I found for photos and images:
The Morgue File Free images for your inspiration, reference and use in your creative work.
EveryStockPhoto.com – Indexes over a million Creative Commons photos for your use.
Wikimedia Commons – The central clearing area for the Wikimedia projects CC files.
Flickr Creative Commons search – Search Flickr for all the derivatives of the Creative Commons licensing.
And a few other I’ve recently had recommended…