Right now, you can't. Nor will be able to in the near future.
Instead, once you upgrade to the new version (which SHOULD be coming out VERY soon if college doesn't get in the way, which is seems like it will and with all the force it can), you'll be able to have several image folders in the same installation, and the configuration panel will let you pick from those, and you'll ALSO be able to configure what image you want to use for the userbar-based signature.
And if you'd like to use a signature that's NOT a userbar, you'd just have to do like my buddy Roa did and create an image with the same text location as the userbar, so no code modification needs to be done.