Xunzian (and others who may also want to do the same) - it’s a simple case of using the BBCode tags in the right order, as they work in sigs just like in posts. For demo purposes I’ll use squiggly brackets {} but square brackets must be used for it to actually work.
To create URL links to other sites (Symposia or otherwise) you put the following:
With brackets this should produce an image that links to the Symposia frontpage.
However (and you knew there’d be a ‘however’) the image is massive, so you may want to download the picture (right click and ‘save image as’) then open up your basic image editing software and resize it, before uploading the smaller version of the same pic to tinypic or imageshack or wherever, and putting the link from that site into your sig between the {img}{/img} tags. If you can’t be bothered with that then just use my own tinypic image link: i14.tinypic.com/4zuoiys.jpg