Change Image Style Depending on User Type

I bet there is a very simple solution to this and I'm just not seeing it for all my searching.

I have a site that displays images with watermarks for anonymous users and I would like to remove that watermark (I think the easiest way would be by changing the image style for the display) for authenticated users.

My question is is there a way to have different displays for different user types? Is this something I should be using Rules for?

Thanks very much.