<!-- IF S_ONLINE -->
Hmm. I guess you could be talking about the notifications display. If so, you'll need to talk to the phpBB devs who wrote the notifications system. The online data might already be there, and if not, the devs might have had the foresight to add a hook to allow extensions to generate online data.
Other than notifications, there is nothing on the default index page where using that image would make any sense. If a user is logged in, they are online! Are you trying to add the image to the output from an extension?