A lot of people like the feature that a folder displays an image of what is inside the folder, but that means you have to keep an image called folder.jpg in the folder.
Another way to go about it is to use a specific icon for the folder. Bút we like it that the image sticks out of the folder, don't we?
Like this:
or 
So here is what we need:
The program IcoFX
I won't go into how it works since the website has it's own section of tutorials and there's no use in reinventing the wheel :)
and empty folder icons:
But - you can't stick images in that stick out ...
To do that you'll need the front and the backside of the folder on different layers.
Here they are:
