Everything on the web is NOT free
Many presenters are under the mistaken impression that they can just copy an image they find using Google Images and use it in their presentation. They are shocked when I tell them that the images are almost always copyrighted and they need permission to use them.
“But the image doesn’t have the little “c” in a circle symbol, so isn’t that OK?” they ask. No, it’s not. If you look at the web site, it almost always has a copyright notice on it, which means everything on the page is protected – including the images.
The common exceptions are government websites where they have usage guidelines posted because by law they have to give some level of usage rights to the citizens who funded the work in the first place. But always check to make sure how you can use it and what acknowledgement needs to be on the slide.
Instead of possibly opening yourself up to legal issues, why not use sources where you explicitly agree to usage, like istockphoto.com and other stock photo sites. And don’t forget that you can take your own photos and have unique pictures that can show exactly what you want.