What you can legally do with an image is governed by copyright law. Copyright law can be fairly fuzzy, but here is what you need to know: In general, use of images for scholarly critique or illustration of a point in a school project should be covered under fair use guidelines; if you plan to distribute or sell your project, you will need to check the use guidelines more carefully. And of course, if you have questions you can always Ask a Librarian!
(Make sure that you give credit where credit is due: cite your sources! See these guides for more help: APA Citation Style Guide, CSE Citation Guide, MLA Citation Style Guide.)
Your computer has the built-in ability to capture a screenshot (an image of what you see on your computer screen). On Macs, press Cmd+Shift+3 to capture the entire thing, or Cmd+Shift+4 to choose a specific section. On PCs, use the "Print Screen" button to capture the entire desktop, then edit the image using the program of your choice.
Online tools:
The Creative Commons Search is a tool to help you locate media on the open web that copyright holders have decided may be re-used for various purposes. CC Search allows you to search Google Images and Flickr.
There are also many online collections of images and videos you may find useful. Here are just a few. Not all items are guaranteed to be under CC licensing, so double-check rights where necessary (see "Copyright & Citing Sources," this page).