That is how you insert an image into a post, however if you notice to the left of the signature box it says:
HTML code is OFF
vB Code is ON
[img] code is OFF
Smilies are ON
while posts are:
HTML code is OFF
vB Code is ON
[img] code is ON
Smilies are ON
So, unfortunately, you cannot insert pictures into your signatures, unless Brandon enables them which I doubt he will do because pictures in the signature can take up a lot of bandwidth for the users with dial-up.