Just like you do in a paper, you need to include an in-text citation in your presentation every time you refer to information not original to you. If it's not your analysis or interpretation, it's probably something you need to credit!
The exact placement of your credit will depend on the arrangement of your content, but remember, you need to be clear what information is coming from what source.
Images that are more decorative than informative don't necessarily need to be cited as formally as information sources. You should, however, be thinking of how you can give credit to your image source. A quick, small note is sufficient.
Copyright considerations: There a variety of places to find images that you can use that follow a Creative Commons license, which explicitly grant you permission to reuse the images so long as you give credit (or possibly a few other criteria). Public domain images are always okay to use. Copyrighted images that you may find through a Google Image Search should follow under the classroom exceptions and/or Fair Use in copyright law, but you should still remember to credit the source.