Creative Commons is a non-profit organization that offers free and voluntary copyright licenses. Creators "retain copyright while allowing others to copy, distribute, and make some uses of their work...."
What Does This Mean for You?
You can search for photos and use those published with a Creative Commons license for free within your presentations. You should check the license to see what restrictions the producer has placed on their content. And, you still need to cite the source even if it has a Creative Commons license. You often cannot alter images that have a CC license.
Several platforms offer images with a CC license. You can search here. But, "Do not assume that the results displayed in this search portal are under a CC license. You should always verify that the work is actually under a CC license by following the link."
Basic Image Collections - Everyone, regardless of topic, should consider searching these excellent image collections.
To find better versions of a specific image, you can use these tools: