The technique for providing rounded corners in tabs consists of 2 copies of the same image "sliding" over each other. As you pull the images appart (one is aligned left, the other right), they will appear as a single image until you reach the max width of that image.
Because of this, you need to create an image that is wide enough to cover the max width of your tab. We limit the width of these images to limit the file size, but you can certainly create a wider image.
Hope this helps,
-Tony