Unfortunately, most people’s first source for images tend to be Google Image search. While effective in finding images, this is not an ethical approach.
Ethics of Copyright
Simple because someone posts material on the Internet does not mean that they are ok with you using them. While it’s incredibly easy to rip off images, it’s not a professional or ethical approach.
Free Clip Art Sources
Microsoft Clip-Art is one of the easiest sources for good artwork. The key is to go to their online website and search for images. These are royalty-free, and do not require attribution.
Another source I recently ran into is StockExchange. Registration is required, and you have to avoid the frequently listing of images from iStockPhoto (which are not free), but this is a great place to find mid- to high-quality artwork.
Creative Commons Sources
The Creative Commons license is a way for people to easily allow others to use their material. The easiest way to find material with this license is Flickr’s Creative Commons Search feature. Go to the flickr site, and start a search. Then, click on the advanced search option.
After getting into the advanced search option, scroll down, and check the “Creative Commons” box. This will only return images that other people have explicitly marked as being ok for you to use.
As long as you provide attribution, you’re generally ok to use any cc-licensed work in your presentations.
Public Domain Images
The last good source of images that I frequent is from Wikipedia. You need to click on the actual image to find its licensing agreement, but generally, images posted there are licensed under the public domain.