I guess you want to show "Whos online" which can be seen at bottom on the index page of phpbb forum..
I think, thats fairly simple, you just have to find out the array, variable where it is stored and then printed in the template.
I don't think thats quite an issue., Yes but U MUST find that variable. ! :-D
If you find and still don't manage to create an image, do send me a post