If you're posting a message using the quick reply or the full reply (forums or pm) or making a new message (forum or pm) or a poll, you may have noticed the new message icon set up.
Basically, just click on the flag and the others will show. Click again on the one you want and it'll replace the flag. Simple, easy!
Now those previously mentioned pages will load faster since only one icon is loaded unless you choose to click for more. Hmmm...now I can add as many as I want too.
[EDIT: Of course, now I just tested the edit feature and it didn't retain the icon I had chosen originally. I guess I need to figure this out.]