Google Images is a great tool to see the work of many photographers. For your presentations, however, the images may not be high enough resolution to project well. It's possible to find useful images with introductory information, and you can even use the advanced settings to limit to works that are in the public domain
Use the build in tools in Google Images to narrow your search. You can limit by size, colors, type of image, date of image, and best yet, you can limit images by usage rights. By clicking on "Tools" from Google Images' search page, you can then see these advanced searching tools. Remember, all images need citations so don't forget to cite your sources!
(Click on image to enlarge)
Did you know you are required to cite images in papers and presentations?
In the body of the paper or on a slide:
More Farmers in Peru Have Stopped Planting Coca, Opting for Cacao and Coffee
Note. From Peruvian Prosperity: From Coca Farmer to Chocolate Maker, by N. Guitierrez, 2016, USAID (https://www.usaid.gov/results-data/success-stories/coca-farmer-chocolate-maker). Copyright 2016 by USAID.
These online repositories provide high-quality images of artwork and other historically significant items that are in the public domain, so you are free to use them in your papers and presentations, but make sure you cite your source!