You'll have to make your div a fixed width. Unless you have mad PHP skills and know how to use ImageMagick (and your server supports it or other on-the-fly image manipulation tools) Then again, I don't really know whether or not that can even do the trick for you. You could try some javascript or PHP to detect browser height/width and have multiple copies of the images at different sizes. When the window is resized, you can poll the window, find out what size it is, and have the browser load the appropriate images.
By the way, I'm just throwing ideas out there, I'm not very proficient with javascript and only minimally so in PHP so I don't know if that could be done, although I would be surprised if it couldn't be done. Easiest to do would be to make your div a fixed width. Less headaches that way
