Tab images are swapped in two states: active and mouseover. The image that swapped is set in the TabParams. You must name your images with an extension for each state, e.g. "pic.gif", "picOver.gif", "picActive.gif".
You would then set the TabParams for imgOverExt and imgActiveExt, like this:
TabParams = {
imgOverExt : "Over",
imgActiveExt : "Active",
...
};
Next in this tutorial: Modifying the TabParams to control the behavior of the tabs