Put your header image/logo into a new <img>
tag that is placed on top of your page. This will give you much more control over positioning than what you're currently doing, and will allow you to use z-index.
If you're comfortable with HTML/CSS, I'd recommend creating a header <div>
and set the background on that div (and also the z-index). This will allow you more options further down the line but the simpler option is to use the <img>
tag.
Here is a jsFiddle with some examples that might help you: http://jsfiddle.net/tMhs7/1/