OK - I was able to link to one of the sigs in Spragueme's photo gallery and use it for her signature.
You right click on the pic and copy what's in the properties field. It ends with aspx instead of jpg, but that's okay.
In the signature area in your profile, use regular html code, i.e. img src="your photo url.aspx" inside the little <> and it should display the image as your signature.
I was doing the [img] thing, which doesn't work in the signature field.