From the provided description, we could only guess that you are using images for the tabs. Still, there are a lot of unclear things. That's why, I think it will be best if you can open a support ticket and send us a simple running project (incl. your custom skin, CSS, images, DB backup if needed and so on) demonstrating the problem (and step-by-step instructions on doing so). In that way we can reproduce and pinpoint the problems you're facing on our side, understand the logic of your application and provide a solution.
Thanks beforehand for your patience and cooperation,
the Telerik team