If its only a few pages that need different images, you could copy/rename the skin and change the image for each variation, then enable per page skins in site settings then specify the skin for each of the pages. You could also structure the locations of your css files so that they are shared across different skins to make it easier to maintain, as described at the bottom of this page: http://www.mojoportal.com/understanding-the-styleconfig-file.aspx
If its more complex than that you could implement a custom UserControl and put it in the layout.master file of your skin and inside the UserControl you you add logic to determine which image to show based on the url.
Hope it helps,
Joe