Ask that trendy coffee shop boy they have on the Mac ads. I though Macs could do anything so easily. At least that's what the ads imply. Nah, just messin' with you. Here is what you do:
So what you do is first off you find an image, and like the other folks said, you stick it in an <img> tag, such as this: <img src="http://www.kittycatonline.com/images/boom3-2.JPG">. That's how you go about it. Note, though, linking to images hosted elsewhere, known as "hotlinking", might not be appreciated, and some sites will block it. The image here is off of my site, so at least today I'm practicing what I preach.