It's kind of tough to spell out because typing the actual tags makes them invisible.... But if I add quotes, you can see them. Just don't type the quotes when you try to imbed an image.
"[img]"http://www.thedoghouse.net/BFG.jpg"[/img]" typed in without the quotes would result in this:
Optionally, I could have just put a link to the image by typing the following
"[url]"http://www.thedoghouse.net/BFG.jpg"[/url]" without the quotes to get this:
You don't actually have to type in the IMG meta tags. You can click those buttons along the top of the post reply screen (just below the subject line field). The first hit of the button results in the starting tag, with the 2nd hit creating the ending tag. Depending on the browser, those tags usually appear at the very end of what you've typed so far, even if you've moved the cursor up the screen.
The tags in phpBB are something that allow people to use html type coding without actually having to know how to code html. And it's a pretty standard format.
And you need to remember that some free picture hosting sites will not let you hot link to the root image or imbed images on other websites. They're advertising driven and want to make sure they get the click-throughs.