Darken background image on hover

Similar, but again a little bit different.

Make the image 100% opacity so it is clear. And then on img hover reduce it to the opacity you want. In this example, I have also added easing for a nice transition.

img {
    -webkit-filter: brightness(100%);
}

img:hover {
    -webkit-filter: brightness(70%);
    -webkit-transition: all 1s ease;
    -moz-transition: all 1s ease;
    -o-transition: all 1s ease;
    -ms-transition: all 1s ease;
    transition: all 1s ease;
}

That will do it, Hope that helps.

Thank you Robert Byers for your jsfiddle

How about this, using an overlay?

.image:hover > .overlay {
    width:100%;
    height:100%;
    position:absolute;
    background-color:#000;
    opacity:0.5;
    border-radius:30px;
}

Demo

If you want to darken the image, use an overlay element with rgba and opacity properties which will darken your image...

Demo

<div><span></span></div>

div {
    background-image: url(http://im.tech2.in.com/gallery/2012/dec/stockimage_070930177527_640x360.jpg);
    height: 400px;
    width: 400px;
}

div span {
    display: block;
    height: 100%;
    width: 100%;
    opacity: 0;
    background: rgba(0,0,0,.5);
    -moz-transition: all 1s;
    -webkit-transition: all 1s;
    transition: all 1s;
}

div:hover span {
    opacity: 1;
}

Note: Am also using CSS3 transitions for smooth dark effect

If anyone one to save an extra element in the DOM than you can use :before or :after pseudo as well..

Demo 2

div {
    background-image: url(http://im.tech2.in.com/gallery/2012/dec/stockimage_070930177527_640x360.jpg);
    height: 400px;
    width: 400px;
}

div:after {
    content: "";
    display: block;
    height: 100%;
    width: 100%;
    opacity: 0;
    background: rgba(0,0,0,.5);
    -moz-transition: all 1s;
    -webkit-transition: all 1s;
    transition: all 1s;
}

div:hover:after {
    opacity: 1;
}

Using some content over the darkened overlay of the image

Here am using CSS Positioning techniques with z-index to overlay content over the darkened div element. Demo 3

div {
    background-image: url(http://im.tech2.in.com/gallery/2012/dec/stockimage_070930177527_640x360.jpg);
    height: 400px;
    width: 400px;
    position: relative;
}

div:after {
    content: "";
    display: block;
    height: 100%;
    width: 100%;
    opacity: 0;
    background: rgba(0,0,0,.5);
    -moz-transition: all 1s;
    -webkit-transition: all 1s;
    transition: all 1s;
    top: 0;
    left: 0;
    position: absolute;
}

div:hover:after {
    opacity: 1;
}

div p {
    color: #fff;
    z-index: 1;
    position: relative;
}

I was able to achieve this more easily than the above answers and in a single line of code by using the new Filter CSS option.

It's compatibility in modern browsers is pretty good - 95% at time of writing, though less than the other answers.

img:hover{
  filter: brightness(50%);
}

<img src='https://via.placeholder.com/300'>

You can use opacity:

.image {
    background: url('http://cdn1.iconfinder.com/data/icons/round-simple-social-icons/58/facebook.png');
    width: 58px;
    height: 58px;
    opacity:0.5;
}

.image:hover{
    opacity:1;
}

JSFiddle