Admin CP -> Appearance -> Templates -> EXTRA.css
Add this code:
Rich (BB code):
.navTabs .tabId a.navLink,
.navTabs .tabId a.SplitCtrl
{
color: transparent !important;
background-image: url('path/to/image.gif');
}
This CSS is a little sneaky but it works. You might have to play with the background position, dimensions, etc.