Using Google Images, perform a keyword search to see relevant results.
Then click on the tools section to filter and improve your results. You'll want to look for an image that has an explicit ok for reuse and make sure the size is medium, or preferably large image. Hover over an image to see its size in pixels.
Note, a PowerPoint slide is 1028 x 768 pixels, so you cannot use a small 200 pixel image without it appearing blurry when projected. Always try to project your images/slides in a trial run to ensure they look good.
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