I think you could do it by disabling the img vb code and then creating a new custom vb code called img, that includes guest parameters. I'm sure there is an easier way, and I'm not positive you can create custom code that uses default tags.
So let me ask this then... what if I just modified the user group for unregistered guests from viewing images by disabling them from being displayed? In theory, that would work... but then how would I go about it? TIA for the advice.