You can use any custom icon for any folder, but, if it's a folder like My images, the custom icon can't show any preview.
ONLY the regular system icon have the dynamic capability needed to show a glimpse of the content.
The point of been able to use an user providen icon it to be able to uniquely identify and personalise some folders when the normal icon don't show any preview, preview is not needed or wanted.
My documents normaly don't have anything to show this way, so, you can have an icon for the tax records, one for business files, one for work files, ...
Why allowing the customisation for all folders:
Some user may need to change the default icons for ANY folder for whatever reason.
Electro
February 2013