The Noun Project is a large library of black & white icons and symbols available to the public domain. These icons and symbols may be useful for your presentation projects, to help teach vocabulary and introduce your topics.
Because they are free (“Public Domain” and “Creative Commons” licenses), you may use these images, but you still need to attribute and use hyperlinks (see what I did above, for the “Global Warming” icon).
If you want to download the images, some you can do without signing up for an account, while some require you to have a (free) account. However, even if you do that, the downloaded images are svg, not jpg or png, and so they’re not very useful for us because we’re all using Weebly or WordPress or Blogger (which don’t allow svg uploads). So what I suggest you do is, as I’ve done, just take a screenshot of the image you want, but just make sure you attribute it. I don’t think this is the best method, but the Noun Project site doesn’t leave us much choice.
If you need help, send me a message on Edmodo or ask in class.