In the initial releases of the RibbonBar, the tabstrip used was RadTabStrip. Yet, the Ribbon own tabstrip was a bit different according to the Microsoft specification. For instance no ImageUrl property or image attachment of any kind.
When we began implementing it more closely we saw this difference and we fixed the tabs. Even better -- we made new tabs that are thin in all aspects -- features, JS, HTML and CSS weight.
That said, the tabs can not be reverted to their previous form.
You could try to work around that by using both CSS and JS, but it will not be an elegant solution. In short you will need to change the look of the tabs to using more CSS instead of images (think CSS rounded corners and not images). Then with JS you need to add span elements in each tab and apply styles to those as well.
If the above sounds OK to you I could prepare a sample snippet for the default skin.
the Telerik team
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