It seems like they're using HTML to display the emoji images. Quite an unique feature though.
It shouldn't be hard, you just have to enable HTML for certain speech bubbles via Habbo UI editor and add some checks if a person executed the :emoji command else sanitize the HTML.
Basically what jerry says but do remember you have to enable HTML client. Two ways to do this is by some coding php allowing html tags to work, or the way Jerry said.