I'm using this code for my overlays and it works every time ^_^

div.pet {background-image: url(IMG URL HERE); background-repeat: no-repeat; width:200px; padding-top:200px} div.pet img {display:none}

Did you put it between style-tags? Without it can't work of course. :)

<style>div.pet {background-image: url(IMG URL HERE); background-repeat: no-repeat; width:200px; padding-top:200px} div.pet img {display:none}</style>