Add a custom icon to OS X folder
The simplest way of doing it is just making it yourself. And there is a quite simple way of doing it.


Select a folder which has the Generic icon.
Do Edit / Get Info or Command+I.
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.
Select the folder you want to change and do the Get info thing as you did before.
Select the folder's image at the top of the info window and paste the new icon by doing Edition / Paste.


This is the final result: 

It's quite nasty, from a designer's point of view, but it will definitely do the trick.