The option I enabled is the URL Rewrite.
It seems to break when gallery is embedded in drupal. The rewritten urls interfere with drupal's node syntax.
phpbb specifically and deliberately ignores parameters in image links to prevent some kinds of abuse.
I'll dig around a little if I get the chance.
Ah. I figured it might be that. The embed is supposed to work with that, Drupal has something similar. But I can't get Gallery to enable URL re-write. There's an option to give the path to the embedded .htaccess file, I did that, made it writeable, and Gallery still errors on it every time. Been frustrating.
At any rate. Posting the link to the image page is preferable in most cases, because it brings people to the site to see the item, which means they may see other items while they're there.
I mentioned before that you don't get the instant gratification of having the image display in the forum, but it's just a click away. And the forum here will continue to load faster without the images linked.