FWIW,
Snopes considers it to be factual.
Keep in mind that as POTUS, and involved in a formal function (that is, the UN meeting) he can't just get up and walk out looking for the nearest bathroom. There are matters of protocol to consider, making sure nobody is offended by his departure. Then there's logistics - his secret service detail isn't going to let him go alone, and they have to make sure the bathroom is secure before he uses it.
I see no reason to doubt the veracity of the photo based soley on the inuendo of a partisan website. It is certainly highly plausable, and Bush has the same bodily needs as everyone else; but has quite a few complicating factors getting in the way of natures call (diplomatic courtisy, securty), and reuiters has a solid reputation.
