I just started using this site but I'm able to see images and think I figured out the rules for joining after trying a few times over VPN. You need a post that follows the following rules:
* Between 20 and 30 English dictionary words (not sure, did not test enough)
* Unique (has not been posted before)
Then it may randomly join you or not (probably to reduce the number of joins). If it doesn't work initially you should try again with new content matching the rule.
see pic