I think you treat a sig pix like posting any other pix, except for sizing.
When i look at my 'edit post signature' theres a bunch of formatting good for my links and between the word Majic and the dates, theres a file attachment ID/code line of formatting.
If I delete just that portion of code, and insert a new photobucket IMG code for a small (50kb+/-) pix, the new pix should be where i put it in my signature, right?
The vB5 software seems to treat signature pix just like a regular pix, you just have to make them small. Otherwise all the same formatting options are there, including using the forum uploader option for pix/attachments.