You are missing display:block;
Code:
.ext-ux-grid-gridviewmenuplugin-menuBtn.x-grid3-hd-btn {
    background-image:url(btn.png);
    width:16px;
    display:block;
}

a.ext-ux-grid-gridviewmenuplugin-menuBtn.x-grid3-hd-btn:hover {
    background-position:-16px center;
}