Particle creates a particle system, where each particle is most often a quad rotated to face the user, but each particle can be any kind of geometry defined by specifying an AssetObject. To set the texture for the quad, set the image_id attribute. To specify geometry other than a quad to use for each particle, specify the id attribute which matches an AssetObject. For each particle, it's initial position, velocity, acceleration, colour and scale can be specified, as well as an additional random attribute which will be added.
http://janusvr.com/docs/build/roomtag/index.html#Particle