Upload an image to represent the folder.
Use the "Browse..." button to select an image from disk.
You can crop the image to the desired result and upload it using the "Apply" button.
The file name will be randomized during upload.
You can also select an image from the list of the already available icons.

The recommended minimum size of the image is 128x128 pixels.
The image will be deleted automatically if this folder is being deleted - unless of course it is still used by another folder.