Have you checked the Error Console in Firefox to see if the image is not loading properly? You could also try checking in Firebug to see whether the image is getting fetched from the webserver. In Firebug, you would check the "Net" tab to view all the resources loaded from the webserver. Your image file should appear in this list.

You could also try running Firefox's built-in web inspector (Ctrl+Shift+I) to check the CSS for that particular page element. You can use this to check that the appropriate CSS classes are being assigned to the GridView.