[danik.js]

<style>
.transition{
    position: relative;
    width: 20px;
    height: 20px;
}
.transition .background{
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    transition: opacity 0.5s;
}
.transition .background{
    background: url(http://javascript.ru/forum/images/smilies/sad.gif) no-repeat;
    opacity: 1;
}
.transition .background.hover{
    background: url(http://javascript.ru/forum/images/smilies/smile.gif) no-repeat;
    opacity: 0;
}
.transition:hover .background{
    opacity: 0;
}
.transition:hover .background.hover{
    opacity: 1;
}
</style>
<div class="transition">
    <div class="background"></div>
    <div class="background hover"></div>
</div>