I'm not a XenForo customer (yet), but I would like to mention that I find it irritating that I don't have permissions to view image file attachments (such as previews of themes) because I am not a customer.
I can understand not being able to download file attachments (zip and what not) which might contain code, but not being able to view attached images when a theme creator has linked them inline is sort of annoying.
There is nothing to fix. Buy a license and get access to the forums that require licensed customer access.
"You must be a XenForo customer to post in these forums. Ensure your account is listed here."
Other paid forum software script support forums do the same thing. Attachments cannot be viewed unless you are a licensed customer, some companies don't even allow viewing threads unless you are licensed.