How to change circular icon to indicate chosen session in unity-greeter?

Solution 1:

Overview

The badges for the Lightdm are stored in /usr/share/unity-greeter.

You can possibly add a new badge to the directory mentioned above. I guess you would want to name it as custom_awesome_badge.png or whatever is the name of the session in /usr/share/xsessions/. All the badges are of 22 x 22 pixels.

From the lightdm maintainer on the ubuntu-devel list:

1. Take /usr/share/unity-greeter/unknown_badge.png and use it as a template (just a 22x22 white circle).
2. Add your logo as a transparent cutout of the white circle (see other badges in that directory for examples)
3. Ship your logo as /usr/share/unity-greeter/custom_SESSIONNAME_badge.png, where SESSIONNAME is your X session name (e.g. 'xfce' for XFCE).

Unity Greeter will automatically look there, falling back to the generic white circle icon if not found.

Set of ready-made custom badges

You can download a large set of additional badges here. To install them just copy the folder content to /usr/share/unity-greeter.