Plus/Minus sign for quantity. Want to change image on hover.

[fiatstudy]

I'm using VM 2.5

The plus and minus buttons that allow you to change the number of items, I want to change images for them when hovered. I wonder what I should add to CSS file.

Tags for the buttons are:
<input type="button" class="quantity-controls quantity-plus">

<input type="button" class="quantity-controls quantity-minus">

Thank you in advance

PS.
This is CSS for the 2 buttons on their "normal" state:
span.quantity-controls input.quantity-plus{background:url(../images/vmgeneral/plus_sign.png) no-repeat; margin-bottom:2px;}

span.quantity-controls input.quantity-minus{background:url(../images/vmgeneral/minus_sign.png) no-repeat; margin-top:-1px;}

[K&K media production]

2.5 is not a vm version

span.quantity-controls input.quantity-plus:hover { ... }