BuddyPress.org

and how do you know or identify who the selected member is?

To do this you would need some means of testing a member for a condition and then based on that condition ‘true’ adding a class token to their listing or ‘a’ tag then you can use a ruleset to add a background image to the anchor or whatever element preferred.

If your not comfortable with PHP, HTML, CSS this is not a easy as it may sound.