How do I make a face of a prim always face the camera?

[CrixusCrucifixus]

I've seen objects inworld that look like a transparent cube with an alpha image texture on one side of it that the image face of the prim is always facing the camera no matter where you walk around. How do I achieve this?

[Rolig Loon]

You are not seeing prims at all. You are seeing particles, which are 2-dimensional textured surfaces.  Torley made a tutorial video that's a little old now but still very informative.  Take a look >>> http://wiki.secondlife.com/wiki/Video_Tutorial/What_are_particles%3F  The video explains how particles are generated.  I suggest visiting the Particle Laboratory, as mentioned in the video.

[Rhonda Huntress]

It is not really a prim.  It is a called a sprite.  It works on the same principal as leases, bling and other partical effects. 

[CrixusCrucifixus]

OK then can i make an alpha image into particles that always face the camera?

[Rolig Loon]

Do as I suggested. Watch Torley's video and then visit the Particle Laboratory in world. It's a lot easier to learn from those sources than it is from a brief explanation here. ALL particles face the camera. That's the way particles work. Yes, you have to create a texture to use for the particles, but the particles themselves are generated by a script that you either need to get from a script library or (more appropriately) write yourself.

[CrixusCrucifixus]

OK then how do I create an always on particle with a texture of my choosing?

[Rolig Loon]

A static particle? You emit one particle at a time with a long lifetime (30 seconds is the maximum) and no motion at all. You'll need to write a script to do that. Take a look in the LSL wiki at llParticleSystem for the parameters. The absolute best way to learn, however, is to spend several hours working through the demos at the Particle Laboratory. That's the way that almost every scripter in SL has learned.