a.button_b_back { display: block; margin: 0 0 0 0; background-position: 0 0; width: 128px; height: 20px; background: url( ../images/button/button_b_back.png ) no-repeat; }
a.button_b_landing { display: block; margin: 0 0 0 0; background-position: 0 0; width: 128px; height: 20px; background: url( ../images/button/button_b_landing.png ) no-repeat; }

a.button_b_back:hover { background-position: 0 -20px; }
a.button_b_landing:hover { background-position: 0 -20px; }