Visuals add punch to your presentations and images are everywhere. Google even lets you do an image search. The problem is that you may be using images that are subject to copyright.
The police aren't likely going to smash in your doors and drag you to jail. The bigger risk is getting caught by the copyright owner and getting a big bill.
If you want to give your presentation in public or to post a recording on YouTube, why take risks?
Stock.XCHNG for years. In recent months, I've started using everystockphoto.com. This search engine lets you search different sites at once — about 15 million images. The advanced options are handy. I normally select landscape photos of at least 500 pixels by 500 pixels from MorgueFile, PhotoXpress and Stock.XCHNG. In the beginning, you may want to use the default settings.
Check the licenses carefully. You might be able to use the images for commercial purposes without giving credit to the creator. As a general practice, I give the creator credit when linking to images online. That feels fair.
If you don't find suitable images, try different keywords. Be a detective.
PaidYou can also get images from commercial sites. You'll get more choice for a price.
SnagIt from Techsmith ($50 US). I use it almost daily for captures and basic editing (e.g., resizing and cropping). That’s how I got the visuals for this post.