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The easiest method for individual folders is to manually cut and paste...

1. Select the icon you want to change your folder to.
2. Choose "Get Info" from the File Menu, or click command-I.
3. Click the icon in the upper left of the info window.
4. Choose "Copy" from the Edit menu (or type command-C).
5. Now, select your folder and "Get Info" for it.
6. Click the icon in the upper left of the info window.
7. Select "Paste" (command-V) from the Edit Menu.

For folder changing fun across your whole system, you could always consider an app like CandyBar.
 
to answer my own question in case anyone else is interested..

you can copy+paste any picture into the info for file to change it's icon, not just another icon. word. enjoy.
 
This method is great in case you want to change a folder icon just once, but if you want to change them permanently, I'd recommend an application called Candybar. That way, whenever you make a new folder, instead of having to copy and paste the image into the Get Info page, it automatically comes up with the new icon.

Downside is that you have to pay for it, I think like, ten dollars? I've been running it in demo mode so I haven't hit that point yet!
 
Candy bar also lets you revert back to the original if you screw up something. Thats what I love about it
 
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