To put an image in your signature, use this code:
[img]http://blahblahblah[/img]
Where it says "http://blahblahblah" you want to replace that with the link of the picture you want to use. If you don't know how to do that, let me know, & I'll explain further.
BBCode is an alternative to HTML. Why it exists, I don't know; it shouldn't, but it does. In posts, whenever you change the colour, make text smaller or bigger, but something in bold, etc., Gaia inserts BBCoding into your post to make those changes.