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!