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!
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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!
Wikimedia Commons contains, according to the site, a collection of over 58 million freely usable media files to which anyone can contribute."Files include images, audio, video, animations, maps, and other multimedia. Users can choose from multiple images sizes when downloading, and each image is often accompanied by information about the work depicted as well as copyright information.
Image Source: "PNG logo with text" by Wikimedia Foundation is licensed under CC BY-SA 3.0
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If you're searching for an image to republish (on a poster or in a paper, for example), you'll need to make sure you have the right to do so. Copyright holders can use a Creative Commons license to assign usage rules and let users like you know what can and can't be done with images. Use the Openverse (formerly Creative Commons) Search page to find licensed images that you can use for scholarly purposes.