The Tabs themselves are actually images and are rendered through the help of CSS Sprites so if you want to you can always just change the sprite to reflect your particular images. You can also play around with this using the Visual Style Builder, it's pretty neat :)
I had tried Style Builder for TabStrip and i noticed that it generate an image from that image it render the tab image, i modified the color combination of that image [multicolor] using photoshop, when i actually testing its rendering wrongly i didnt changed the css even the image name is also same, i just modified the colors of that image.
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if i want to use my customer images then let me know the procedure