.image {
    background-image: url("image.jpg")
}

<span class="image" title="This is a hint.">

.image:hover:after {
    content:"This is a hint";
}

<span class="image">
    <span class="hint">This is a hint</span>
</span>​​​​

.image .hint {
    display:none;
}
.image:hover .hint {
    display:block;
}