Have you tried phpbb 3.1.4 yet you can just drag and drop the pictures in the box where you type a reply to a topic (same box I am typing this reply) and it automatically uploads the pictures NO MORE selecting each attachment one at a time then clicking upload, having that annoying box popup then when its done uploading the single attachment reloading your page so you have to scroll down so you can add your next attachment.
I can add 20 pictures to a post as fast as it takes me to attach just one picture using phpbb 3.0.12. just open up Explorer (in Windows) navigate to where I keep my pictures select the pictures I want to attach to the post drag and drop them DONE.
The way phpbb 3.1.4 allows you to attach attachments is genius ! Give it a try I doubt you will still want to use something like vbulletin 3 style attachment uploading...
My hat is off to the phpbb development team job well done!
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