You do not need stretch-top, stretch-bottom, etc. there.
Please see: http://www.terrainformatica.com/wiki/doku.php?id=h-smile:expandable-backgrounds
Change your style to:
/* start button */
.startButton{
font-size: 14px;
width: auto;
margin-top: 10px;
padding: 10px 20px;
background-image: url(images/btn.png) ;
background-position: 9px 6px 9px 6px;
background-repeat: expand;
}
.startButton:hover{
transition: blend;
cursor: pointer;
color:#669900;
background-image: url(images/btn_hover.png);
background-position: 9px 6px 9px 6px;
background-repeat: expand;
}
stretch-bottom/top for parts containing transparent areas are not working quite well now. But you do not need them there anyway.