Folder Icons and .icns files

I came across a problem when changing a folders image. The folder looks fine when the folder size is above 32 pixels, but once the folder size is reduced to 32 pixels or less it doesn't look the same as the default folders. The default folders look like they have been outlined so you can see what's on the folder. But when you just copy, lets say a 512 pixel image, when you reduce the size of the folder it doesn't look the same.

It looks like this..

(When a 512 pixel image is copy then pasted for the folder's icon.)

And this is what it looks like on the .icns file. (How it's suppose to look)

So I want to know how to use a .icns file as the folders icon since there are several size images located within the folder.