First, header image - probably because it's the biggest image on the page. Text loads quickly, images take time, the bigger the longer. Anyhow, for me (hi-speed connection) it loaded at the same time as the rest of the page anyway - no problems.
Second, with regard to your header bg image (assuming you mean the background behind "menu header"), here's what you put in your code:
border:1px solid #DEE3E7;
That span tells me (and CSS) that you want a span behind the text "menu header" with a certain background - not that you want a child division filling a certain area of a parent division, regardless of what text you put on top of it. If you change your text to "the menu header goes here", the background (as it is now) will just expand to fill in the area behind the text b/c that's all you've told it to do, so far...