To download the clip art and animated GIFs on this site, just move your mouse over an image or animation and right-click. Then select "Save Picture As..." (or "Save Image As" if you're using a Mac). Feel free to open the pictures in your favorite graphics software, such as Photoshop, Fireworks, Gimp, etc... and edit if you wish. Keep in mind that linking to these images is not permitted.
If you mean your own artwork, then you'd take a picture of it, upload it to your computer (make sure you know where you're saving it), then select Insert > Picture in Word, browse in the location where you saved the photo, click the picture, and insert. If you mean an image from the internet or something, you can just save it to your computer and basically follow the rest of the steps above.
This applies more to teachers than to anyone else, but clip art (especially black line clip art) can´t be beat when it comes to using images that are going to be photocopied. It creates a much cleaner look than a photo. While there is much talk of going digital, the reality is that most of the world still is using paper for teaching . . . and a lot of it, not because we aren´t open to the digital world, but many times not all students have access.