If you have a fixed height in your container, you can set line-height to be the same as height, and it will center vertically. Then just add text-align to center horizontally.
Here's an example: http://jsfiddle.net/Cthulhu/QHEnL/1/
EDIT
Your code should look like this:
.img_thumb {
float: left;
height: 120px;
margin-bottom: 5px;
margin-left: 9px;
position: relative;
width: 147px;
background-color: rgba(0, 0, 0, 0.5);
border-radius: 3px;
line-height:120px;
text-align:center;
}
.img_thumb img {
vertical-align: middle;
}
The images will always be centered horizontally and vertically, no matter what their size is. Here's 2 more examples with images with different dimensions:
http://jsfiddle.net/Cthulhu/QHEnL/6/
http://jsfiddle.net/Cthulhu/QHEnL/7/
UPDATE
It's now 2016 (the future!) and looks like a few things are changing (finally!!).
Back in 2014, Microsoft announced that it will stop supporting IE8 in all versions of Windows and will encourage all users to update to IE11 or Edge. Well, this is supposed to happen next Tuesday (12th January).
Why does this matter? With the announced death of IE8, we can finally start using CSS3 magic.
With that being said, here's an updated way of aligning elements, both horizontally and vertically:
.container {
position: relative;
}
.container .element {
position: absolute;
left: 50%;
top: 50%;
transform: translate(-50%, -50%);
}
Using this transform: translate(); method, you don't even need to have a fixed height in your container, it's fully dynamic. Your element has fixed height or width? Your container as well? No? It doesn't matter, it will always be centered because all centering properties are fixed on the child, it's independent from the parent. Thank you CSS3.
If you only need to center in one dimension, you can use translateY or translateX. Just try it for a while and you'll see how it works. Also, try to change the values of the translate, you will find it useful for a bunch of different situations.
Here, have a new fiddle: https://jsfiddle.net/Cthulhu/1xjbhsr4/
For more information on transform, here's a good resource.
Happy coding.