I'm a fan of using Google Image Search to find pictures for my PowerPoint presentations. But there are legal limits to what I can use -- defined by the license on the photo -- so I need to be careful. For a long time Google just slapped a generic warning on the search results that alerted you to copyright issues. Caveat Emptor, or something like that.
Bing -- Microsoft's most recent search engine attempt to dethrone Google -- has had the ability to search images and filter the results by license since December, but now you can do the same thing in Google.
When you do an image search in Google you can filter the results by clicking on the "Search Tools" button above the results. From there you can filter by image size, color, type, time and now license type.
If you aren't getting paid for the use of the image you can use the least restrictive license type: Labeled for Reuse. From there the restrictions add up, you can have something labeled for reuse with modification, for commercial reuse or for commercial reuse with modification.
So if you want to edit a photo to use in selling your next book, you'll need the images licensed for commercial reuse with modification, but if you're giving a presentation at a non-profit and you don't plan to edit the picture you can use the images licensed for reuse.
Here are some other presentation resources that might be helpful.