If you're in a room, press the 'My Account' button. A new page will open for you.
Go down to Media Vault, select Public Image Gallery.
Peruse through the icons, or pictures, which are grouped into subjects.
Select one, press the 'Add to my Account' button.
When you go back to the room, refresh the room.
The picture, or icon will be in the pull down menu titled chat picture.