Smilie code to parse BBCode?

Odd question, but hopefully someone knows the answer. I'm looking to make a smilie code, post and parse a BBCode. For example, if someone posted:

:ducks:

The replacement would be:

[img] url_to_a_picture_of_ducks.jpg [/img]

I tried using a replacement variable, which does what it should, but vB doesn't parse the image code - it just shows up as is. If I edit/save the post it displays the image, but I need to add that second parse in there so that when a user types :ducks: both the replacement variable AND the IMG call happen at post time.

Is this possible?