How to format the "Selected" portion of a button

I created a simple menu screen with 4 chapter buttons (a jpeg for each chapter) and I'm trying to format each one so that it is highlighted when selected. Problem is, the highlighted area is significantly larger than the jpeg it is supposed to cover and actually bleeds onto its neighboring buttons. When I try to size it down, the jpeg shrinks along with it! Also, there's a strange little line that appears at the bottom of each jpeg when selected (it looks like a thin button, but there is no way to delete it or format it either).

I tried going through the DVDSP manual & index but nothing seems applicable.

Is there a way to format the "highlight" on the menu button/jpeg so that it only covers the jpeg?

Thanks for any assistance!