You cant use vb3 variables in photopost pro and usually if a member can view a medium image they also can view the large image there is no permission distinction between those images because they are the same images. If a usergroup has permission to view images in the category in question they will see the images.
We can only support our code as it is written. You are free to discuss your code modification ideas with other users here if you wish. http://www.photopost.com/forum/photopost-pro-mod-discussions/