Are you asking for something like "Actually online: 55"?
If yes - there's already something like that on the bottom of the main gallery page.
If no - you can always implement an external hit-counter on the page - that would be the easiest way...
But: I would be also interested in a simple statistical overview like:
VisitorsThis month: 3500
This week: 1000
Today: 100
On-line: 10
Would that be possible?
I have something like that done by an external statistics-provider, but it is not very realistic - the stats-server is often down