It is a much better idea to use full-sized photos that bleed*. I would suggest using your own photographs when possible, since it adds a personal touch to your slides. However, you will probably often find yourself in situations where you cannot take that perfect shot or just don't have time to set it up.
The next best thing (or, alternatively perhaps, the best thing) is to use royalty-free stock photos. If you are not familiar with the terminology, royalty-free is "right to use copyrighted material or intellectual property without the need to pay royalties for each use or per volume sold, or some time period of use or sales."* It can be purchased or free of charge.* "Stock photography is the supply of photographs licensed for specific uses. It is used to fulfill the needs of creative assignments instead of hiring a photographer."*
Recently finding free royalty-free stock photos is becoming more difficult. However, here are two sites presenters often check for free photos:
Flickr
EveryStockPhoto
With Flickr, be sure to check the license. Many photographers DO NOT give CC licenses. You can also search Flickr specifically for CC licensed photos.
As you can imagine, commercial photography is often of greater quality. Additionally, sites that carry commercial photos tend to have better organization and better search tools. The downside is that the photos cost money and often are very expensive, especially if they are for slides. One site with some of the most reasonably-priced high-quality photos is iStockphoto. One nice feature of this site is that when you register, you are able to download one free high-quality photo every week (there are also free, weekly or monthly videos, illustrations, and audio downloads). Not all files are priced the same, so be sure to browse and check out the different photographers.
iStockphoto.com
If you have any questions, comments, or other experiences to share, please do so in the comments section. I'd love to hear from you!
More Reading
*Bleeding on Wikipedia
*Royalty-free on Wikipedia
*Creative Commons licensing on Wikipedia
*Stock photography on Wikipedia
