The signature you mean? The panel to edit it is pretty much the same as your regular posting panel and obviously you know how to customize the color already. :]
To put in a picture, copy the image url and paste it the brackets you get from clicking the button that says Img.
It should look something like this in the coding:
[img]URLhere[/img]
For changing the font, I've never done that myself (I only play with size, bolding, and Itallics) so I'm not positive, but you may need to go off site for that. Hopefully someone who knows better than I will come along to help you. :]
edit: The Hangul (funky characters) in my siggy is copy-pasted from a lyric site. I'm fairly certain copy-paste is how most people do it.
Also, just for future reference, questions like this go on
this board. It'll get more attention there as well. :]