It used to be, you were limited by what came with Word for your clipart and images. There are now several quality royalty-free image sites. I'll go over my favorite, stock.xchng. They do require registration, but it's free and spam free.
Go to http://www.sxc.hu and search for a topic (for this example, I'll do 'science'):
This will bring up several results. All of the images displayed are free. Except for the bottom of the first few pages. You can tell which ones cost money when they look like this:
Of those, even some are free. They just ask that you give them credit or link to their home page. If you like an image from the 'Premium' results, just click on it and it will tell you the cost.
When you find an image you like, click on it. This will bring up the picture's information page:
On here, you can see who uploaded it, a link to the author's website, and how many times it's been downloaded. At the bottom, it will show you the size (most are very large and will need to be resized before using), and if there are any restrictions on using it. If you decide you do want this picture, click on the Download button. This will display that image full size. Right-click on the image and click Save Image As...
The image is now saved on your computer and ready for resizing or using on your website or document.