You'll need to host your images first with
tinypic.com who I prefer. My logos sizes are 280 x 100 which are ideal sizes.
As I've got 3 logos I had to shorten the url's to fit in the signature box so used google URL shortener but if you want just one logo you'll be fine with the original URL's.
Code is [*url=http://Your Domain Here][IMG]http://Your Image URL Here[/img*][/url] (Delete the stars)
Here's one for you Trev
[*url=http://goo.gl/GESPu][IMG]http://goo.gl/lwJcy[/img*][/url] (Delete the stars) This one is already shortened with
http://goo.gl/ where you can track number of clicks. Well as I created it on my Google account only I can see the click number, if you create it you can see it.