If you try to post an image, wrapped in url tags, and with text on the line below, the url bbcode will not be parsed. I've tested this on multiple phpBB3.0.2 installations, including the one found at the official phpBB community.

Here's an example:

[url=http://thebrawl.net/signaturegenerator/][img]http://thebrawl.net/signaturegenerator/luigi/mrsng_Daniel%20St.%20Jules_2363%205328%206566.jpg[/img]

Text below image which should still be displayed within the link[/url]

And the outcome:

[url=http://thebrawl.net/signaturegenerator/]!http://thebrawl.net/signaturegenerator/luigi/mrsng_Daniel%20St.%20Jules_2363%205328%206566.jpg
Text below image which should still be displayed within the link[/url]

I'm assuming that this might be an issue with the inserted line break.