Here's how it works. I've tested this on phpbb.com to ensure I'm not cray...
An img tag on it's own, has no behavior. Clicking on it does nothing:
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[img]https://i.imgur.com/PzDxBKe.png[/img]
The resultant HTML is:
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<img src="https://i.imgur.com/PzDxBKe.png" class="postimage" alt="Image">
An img tag surrounded by a url tag, where there is no specified url tries to go to the text within the url tag. This will NEVER work:
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[url][img]https://i.imgur.com/PzDxBKe.png[/img][/url]
The resultant HTML is below. The <a> has an href to the encoded text that is within the url tag, and it behaves exactly as one would expect (badly):
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<a href="%5Bimg%5Dhttps%3A//i.imgur.com/PzDxBKe.png%5B/img%5D" class="postlink"><img src="https://i.imgur.com/PzDxBKe.png" class="postimage" alt="Image"></a>
What you've seen work before is below. Where the url tag is used properly, by specifying the target with url=. Almost all of my posts, where I use Flickr, have this done correctly:
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[url=https://i.imgur.com/PzDxBKe.png][img]https://i.imgur.com/PzDxBKe.png[/img][/url]
The resultant HTML is:
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<a href="https://i.imgur.com/PzDxBKe.png" class="postlink"><img src="https://i.imgur.com/PzDxBKe.png" class="postimage" alt="Image"></a>