LARP is what happens when you get a bunch of Dungeons & Dragons fans....who also have a penchant for Cosplaying.
Since the uniforms in that photo look very Roman Legionaire-like, the joke centers on the concept that someone in Persia is forcing them to do a Roman LARP thing.
The 30 second post limit is an anti-spam feature. 90% of the time, anyone who's posting more than twice per minute is either doing so to be disruptive, or should edit the first post rather than rapidly post a second. This is a pretty standardized feature in all decent Forum/BBoard software.