A signature is a little text that can be added at the bottom of your posts. It's limited to 1000 characters (size 8 to 14) on 6 line(s) and 5 image(s) with none larger than 80x80 pixels and for a maximum of 80Kb. In your text, no more than 100 characters without space too.
Now, I'm not one to complain - but I think the 80x80 pixel limitation would exclude pretty much anything other than small icons and member's logos. Heck, I can't even add the wee .gif file I use to 'sign' my name! Any chance of getting this increased a bit?