#btn
{
     height: 29px;
     width: 77px;
     border: none;
}
#btn.play {background-image: url("play.gif"); }
#btn.pause {background-image: url("pause.gif"); }

<button id="btn" class="play"></button> // Does Display
<button id="btn" class="pause"></button> // Doesn't display

---------------------------------------
#btn.pause {background-image: url("pause.gif"); }
#btn.play {background-image: url("play.gif"); }

<button id="btn" class="play"></button> // Doesn't Display
<button id="btn" class="pause"></button> // Does display