I have several theme skin files that a user can select from. this all works fine.

I have gridviews on some of my pages that have a div above them, as a header. I use an image for background color with the div's. I would like to change the image used for the div background based on the theme skin the user selects.

How can this be done?

I would believe that the best option would be to create a corresponding CSS file that contains a class for your div that includes your background-image setting.  You can use this same format for your multiple themes.

You could also try to add it to the skins file by using <div runat="server" style="background-image:url(...)">, but I'm not sure if this will work.

Another option would be to use a Panel control in place of a div.