You can use images you find on the web; but remember the internet is not a sentient being that creates things. Text, images, graphics, and photos found on the internet have an owner. You can legally use these materials in one of three ways. Sometimes that owner is gracious enough to share his or her creations using a Creative Commons License.  Second, you can ask permission from the owner. Last you can purchase the photos from a stock photo site such as iStockPhoto. 

It is good practice to credit any images, graphics, or photos created by others that you use in your presentations. This is a simple courtesy and a way to avoid any possible disputes over ownership. Even if an image is a screenshot, it is good practice to note the image as such. So don't forget to give credit where is credit is due, or in other words attribute. 

This humorous blog post provides guidance: Adding photo creds to talk slides.

This infographic from the University of Illinois asks and answers: Can I Use That Picture?

Finding Free Images Using Creative Commons

Creative Commons licensing is inspired by open source and the GNU Project approach to software licensing. It allows photographers and creators to select which permissions to give to people, for free. Creators can select from a number of licensing options. The most common licensing is an Attribution license, which lets anyone use your image in any way they like, as long as they give you credit. Which seems fair enough. All you should have to do is include a link back to the original page and to the creative commons license. 

	Pixabay is a good option to use to locate for images. On Pixabay, you can find quality images free of copyright restrictions with a simple registration. Creators waive their copyright and related image rights under Creative Commons deed CC0, thus permitting free usage of their work. This allows you to use, modify and redistribute the images freely - even in commercial applications - without asking for permission and without attribution. Despite this, it is common courtesy to acknowledge the creator in an attribution line or note in your presentation.
	Flickr is another good source for images. Flickr offers creators the ability to either release their images under certain common usage licenses or label them as "all rights reserved." Many images are licensed under the Creative Commons 2.0 attribution-based and minor content-control licenses.  You can limit your search by license type.
	Microsoft's Office Suite once had an online gallery full of free stock images. In December 2014, this library was merged into Bing Images. Now you can compose a search in Bing images, and select your use privileges. You can set either the Creative Commons license, free to share and distribute commercially, or free to distribute and share commercially.

You've located an image but don't know who to credit. There are  reverse image tools that you can use to identify images on the web. Generally you upload a copy of the image and the tool identifies the image.

TinEye is one of the image identifying reverse image search engines. You simply upload an image to TinEye to find out where it came from, how it is being used, if modified versions of the image exist, or to find higher resolution versions. TinEye is the first image search engine on the web to use image identification technology rather than keywords, metadata or watermarks. It is free to use for non-commercial searching.