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This solution from Apple replaces the generic, blue folder with the specified graphic element.
Is there a way to add a custom graphic 'over lay' to the generic, blue folder? Dropbox is an example of this.
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I want to do this for one folder. An alias to the folder should retain the icon as well.
I would prefer to do this without the help of a 3rd-party application.
The simplest way of doing it is just making it yourself. And there is a quite simple way of doing it.
Select the folder's icon, at the top of the Info Window, and copy it.
Open Preview.app
Do File / New from Clipboard or just Command+N. All of the actual icon's image sizes will appear. Select the first one as it's the biggest one. 
Copy, from elsewhere, the image you want to use as overlay and paste it in the Preview document. For pasting it you'll have to click on the actual image, instead of Preview's selecting panel. 
Now, either after clicking in the preview's panel or the actual image, do Edition / Copy.
This is the final result: 
It's quite nasty, from a designer's point of view, but it will definitely do the trick.
You can do this with Folders Factory:
Contacts might not be the best image for this, but you get the point.
You can also just overlay an image, or stamp a texture or text, but based on the Dropbox I guess this is what you're going for.
