Disclaimer: OK - so I know I shouldn't be using tables, but sometimes, I just have to.
Was trying to figure out this extra space around an image that was sitting in a table cell. Came across this article which explains why.
It's to do with using a strict doctype, and the fact that the images are by default inline which display on top of the baseline. Quick fix for me was to apply the following style to the offending images:
.header-image {display:block}