You can save them as png files. I saved the ones I did for my CT as png files cuz I wanted the transparent background. I was a little worried that they wouldn't show up right, but they show up just fine. Of course if you don't need the transparency... saving as a jpeg will make the file size quite a bit smaller. You can also save them as a gif files (but your shadows will get a bit funktified).
I made my entire team a little siggy tag. They aren't overly exciting since I was trying to keep them on the small side. I have my blog set up to have my siggy show at the bottom of each of my posts. My team members who post on the blog have the same thing. Very cool!