How to make in CSS an overlay over an image?

You can achieve this with this simple CSS/HTML:

.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);
}

HTML

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

Demo: http://jsfiddle.net/6Mt3Q/

UPD: Here is one nice final demo with some extra stylings.

.image-container {
    position: relative;
    display: inline-block;
}
.image-container img {display: block;}
.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);
}
.image-container .after .content {
    position: absolute;
    bottom: 0;
    font-family: Arial;
    text-align: center;
    width: 100%;
    box-sizing: border-box;
    padding: 5px;
}
.image-container .after .zoom {
    color: #DDD;
    font-size: 48px;
    position: absolute;
    top: 50%;
    left: 50%;
    margin: -30px 0 0 -19px;
    height: 50px;
    width: 45px;
    cursor: pointer;
}
.image-container .after .zoom:hover {
    color: #FFF;
}

<link href="//netdna.bootstrapcdn.com/font-awesome/4.0.3/css/font-awesome.min.css" rel="stylesheet"/>

<div class="image-container">
    <img src="http://lorempixel.com/300/180" />
    <div class="after">
        <span class="content">This is some content. It can be long and span several lines.</span>
        <span class="zoom">
            <i class="fa fa-search"></i>
        </span>
    </div>
</div>

You could use a pseudo element for this, and have your image on a hover:

.image {
  position: relative;
  height: 300px;
  width: 300px;
  background: url(http://lorempixel.com/300/300);
}
.image:before {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
  transition: all 0.8s;
  opacity: 0;
  background: url(http://lorempixel.com/300/200);
  background-size: 100% 100%;
}
.image:hover:before {
  opacity: 0.8;
}

<div class="image"></div>

Putting this answer here as it is the top result in Google.

If you want a quick and simple way:

    filter: brightness(0.2);

*Not compatible with IE

A bit late for this, but this thread comes up in Google as a top result when searching for an overlay method.

You could simply use a background-blend-mode

.foo {
    background-image: url(images/image1.png), url(images/image2.png);
    background-color: violet;
    background-blend-mode: screen multiply;
}

What this does is it takes the second image, and it blends it with the background colour by using the multiply blend mode, and then it blends the first image with the second image and the background colour by using the screen blend mode. There are 16 different blend modes that you could use to achieve any overlay.

multiply, screen, overlay, darken, lighten, color-dodge, color-burn, hard-light, soft-light, difference, exclusion, hue, saturation, color and luminosity.