I tested two images but when I attempt to post the topic in the gallery forum, it halts with a message
The host from which you are accessing the forum is not recognized as a valid host. This is more than likely related to a firewall issue that is blocking the referer variable. Check your firewall settings and try again.
As firewalls can block the referrer value from populating, it is not recommended to have the referrer check enabled (as it should be disabled by default).
Please ensure "Disable HTTP Referer Check"
is selected:
CP -> Master Settings -> Primary -> Basic Tab -> Advanced Settings -> "Disable HTTP Referer Check"
From the notation: "Unselecting this item may cause login and posting issues for your members. We recommend keeping this box checked unless you have a special need for this feature."