OK, if I understand, tags are removed if there's no data. That's normal behavior. What I've done is inserted some text between tags. For example (to use your example):
To post an image, you surround the image url with [img]and[/img].
And:
All image modifiers in this section can be used with the avatar tags. Here we'll use [img]<img_url_here>[/img], but you can modify any of the following with them, as they are shortcuts to image files; [av], [ghost], [robot], [selfie], [outfit], and emotes.
Anything other than a <space> can be inserted in there like a <period> [img].[/img], but one work-around is to insert a smilie between tags and it gets the results you're after. The problem is if you edit the message, you'll lost the tag. [img]
[/img]