I assume you've seen the chat systems available in the Asset store (a good place to start). A quick and dirty way would be to embed them in the font texture, then detect and convert the text emoticon like ;) to the character code for that symbol in the font texture. To get fancier, like animated ones, you could either have multiple such characters that you cycle through, or get down-and-dirty with the API to use DrawGUITexture where needed. For that you would need precise font metrics. See GUIStyle.CalcSize and related functions.