How do I put a hover with a background-image in a ToggleButton

I have put this in the CSS

.image_atras span {

background: url('images/btnAtras.jpg');

background-repeat: no-repeat;

width:30px;

height:28.5px;

margin-right: 10px;

float:right;

}

.image_atras span: hover {

background: url('images/btnAtras2.jpg');

background-repeat: no-repeat;

width:30px;

height:28.5px;

}

But when It hovers the image desappears and none image appears