same way you put them in a post, with the <img> </img> tags. Just use the square brackets [ ] instead of the < > ones.
example, <img>
http://www.urlofpicturegoeshere.com</img>
the limit is 100K file size, and 500 pixels wide and 160 pixels tall. Of course if you're gonna add a pic that size it would be a good idea to scrap those little images. This limit is meant to save:
a. the 56K people from having to download huge sigs all the time, and
b. to save people from having to scroll past huge sigs every time you post.
Please keep within these guidelines as not everyone is on cable, or uses a 1024x768 resolution. These aren't my guidelines, they're the BuF admins, so please keep to them. Thanks.