How to do it? That's HTML stuff. You got to write the following:

[img src="address of picture"]

Note that you should use the pointy brackets <> instead of the square ones I've used. I didn't use it now because the browser would mistake it for HTML and display an error.

The pic should be somewhere on the web. Suppose while you're browsing, on a particular webpage you find a pic you'd like to put here, right-click the pic and click 'Properties'. You'll find the address there. Select the address with your mouse, press CTRL+C to copy, and paste it in place of "address of pic" above. Try Vincent's pic below. You'll find the address is "http://imagecache2.allposters.com/images/20/V113VANG.JPG".

So, all you got to do is type
img src="address of picture"
within pointy brackets and post it. You'll find the picture on the blog.