Hello François,
As you have noticed, the use of only one image in the RadTabStrip skins is due to a significant performance boost because of the
reduced count of http requests. This technique is called
image sprite and can be described as "old school" and extremely efficient. If you are interested, I recommend the
"CSS Sprites: Image Slicing’s Kiss of Death" article at the
A List Apart ezine. We do our best to use it across all our controls and you'll see it more often in the future. And of course, because of the increased complexity of the skin modifications, we are working simultaneously towards easier ways of adjusting the skins... but this is only a teaser, so don't mind it ;-)
All the best,
Paul
the Telerik team