Shoutbox

It is a custom emoticon, either you, or someone on the computer, added it to messenger.

To change it click on the emoticon button in a chat window, then press "Show All"  Then scroll down to the face that has "yes" written beneath it, then either click "modifiy" and change the "yes " to "(yes)" or something that will not interrupt normal typing, or select "remove" to delete it completely