I need the image positioned behind a div (but not the main div) containing the text.
Its kinda
[SCREEN SHOT INCOMING]
Like that, except imagine half the image behind the 'text' and half out.
I think you would need to make sure the image is contained in a div, then you have to specify "position: absolute;" within that div's css, which gives you ability to specify an exact position. I think somewhere there is a 'z' attribute you can set to specify the 'depth' of the div to put elements ontop or behind if need be.
Sorry I can't be more help. I develop Intranet sites if anything in asp.net so rarely have a need to deal with funky CSS stuff. I leave that to the younglings.
Syaynay is right. You would want to take the image out of the natural flow of the page by setting it to position:absolute and then set the location of the image. You can use the z-index attribute to tell the browser which elements should be on top of each other.