You don't put [/SIGPIC][/SIGPIC] around a photo. You put [IMG][/IMG]
Example:
Here is an image link:
http://cdn.stereogum.com/files/2009/07/muse2009.jpg
Now by putting [IMG][/IMG] around it, it will embed.
You can upload the image to a site like photobucket or imageshack and have your own link to it, and then do what I did but in your sig. I find it easier than uploading the image from my computer. Had too much trouble doing that.