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<div class="image-container">
    <img src="http://lorempixel.com/200/300" />
    <div class="after">This is some content</div>
</div>


CSS:

.image-container {
    position: relative;
    width: 200px;
    height: 300px;
}
.image-container .after {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    display: none;
    color: #FFF;
}
.image-container:hover .after {
    display: block;
    background: rgba(0, 0, 0, .6);
}