Some areas like "xenforo plugins" do show the screenshot avatar, but a visitor without an active license can not see the bigger picture. My suggestion is to display the big image for all visitors. But restrict access to the code.
It is a selling point to have screenshots available for the third party products available for xenforo.
Currently, downloading attachments is a binary permission which doesn't differentiate between pictures and other types. How permissions work for attachments will need to be changed before non licensed members, and guests, will be able to view screen shots. There is no plan to do so at this time.