My guess is the forum design leaves a fix gap underneath the avatar & location - if the resulting total height of the avatar / location 'box' is greater than the height of the body of your post combined with your sig height you'll have a gap under your sig.
If however you have a huge sig or yours is a long post and so their combined height is greater than the avatar / location area, your sig will end flush with the bottom and you'll have a large gap under the location.
Hope that made sense... it is just a guess too...
bartboy9891 - To add pics to your sig use img tags the same way as you do in a post.