A typed chat works like this: Each time you or your chat partner types something and then presses Return, the text appears on both of your screens.
Messages displays each typed comment next to an icon, which can be any of these three things:
A picture the other person added. If the buddy added her own picture—to her own copy of Messages, a Jabber program, AOL Instant Messenger, or Yahoo Messenger—then it will be transmitted to you, appearing automatically in the chat window. Cool!
A picture you added. If you’ve added a picture of that person to the buddy list or to Contacts, you see it here instead. (After all, your vision of what somebody looks like may not match his own self-image.)
Generic. If nobody’s done any icon dragging of any sort, you get a generic silhouette icon.
To choose a graphic to use as your own icon, click the round picture to the right of your own name at the top of the buddy list. From the pop-up palette, you can choose, as your avatar, any of Apple’s Defaults (nature shots), Recents (chat photos you’ve used recently), My Photo Stream (your own recently taken pictures), Faces (if you’ve set this feature up in iPhoto), or Camera (take a new picture right now).
In most cases, you’re offered an Edit button. It opens a little adjustment window where you can fiddle with the size and centering of your headshot.
Feel free to build an array of graphics to represent yourself—and to change them in midchat, using this pop-up palette, to the delight or confusion ...