Slides and Copyright Infringement
If you're going to use slides in your presentation, you need to be aware of copyright infringement. While it's true that images are usually better than words, there's a big problem with images ― you should only use them on your slides if you have permission to use them!
It comes as a surprise to many people that photographs can be copyrighted... but they frequently are. So are most songs and most comics. So before you go using your favorite "Peanuts" cartoon in your slideshow or playing "The Wind Beneath My Wings" during your presentation, it is your responsibility to be sure that you have permission to use them. Not doing that is copyright infringement ― or to put it another way, theft.
You cannot legally use copyrighted resources unless you get permission from the owner of the copyright. (The exceptions are resources that are in the public domain or are clearly licensed under Creative Commons for your use ― the song "London Bridge is Falling Down", for example, is in the public domain because of its age.)
But back to photos. Many photographs are copyrighted, just like cartoons or songs. Let's debunk a popular misconception ― just because you found a photo on the Internet doesn't mean it's in the public domain. People can oftentimes put something on the Internet without a problem because it's for their own personal use. That's legal. But when you use something ― a photograph, a cartoon, or whatever ― in your presentation to show other people, it's no longer for "your personal use" and copyright law applies.
Now you may protest that you know of people who used photos (or cartoons or whatever) in their presentations and they didn't have a problem. In practice, if you're giving a presentation to a small local group and there's no money changing hands, quite possibly no one will care if you don't have the proper permissions to use copyrighted material. (However, it's still illegal.)
But if your presentation is being seen by a larger group of people or if you're getting paid for your speech, then you should definitely not use copyrighted materials ― photographs, songs, cartoons, whatever ― in your presentation. You're breaking the law and if anyone catches you, the consequences can be serious.