Hi, I'm Andy nice to meet you. My favorite image host is imgur.
I've worked since 2010 with phpBB, done conversions from phpBB 2.x to 3.0.x and 3.1.x, different mod installations and writed custom mods and integrations with various websites.
I can have a look on your forum, I think we can update your forum to the last version of 3.0 branch(3.0.14). After this phpBB changed their core and any modification before will become incompatible, and would be possible only if you agree to give up on your current mods.
About the image mod, I'll create in acp a settings page where you can add hosts, whitelist, blacklist those, all is clear.
I've done few years ago a mod related to the image tag and bb tags, so I have all the experience I need.
Hopefully I'll hear soon from you.
Regards,
Andy.