You need just to edit the images, there shouldn't be a problem with the CSS positioning in such case.
By the way we are currently working on a new blog post where you will find more information about RadWindow's design and structure - it should be available either tomorrow or on Monday.
In the meantime if you experience any problems with the sprites, just open a support ticket and send us a project that illustrates them - we will check it right away.
the Telerik team
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