If it's out of the holster it's highly likely that it's the 'sling' gun from the pre-production Fett costume.
It's a short pistol with a 'stock' that goes back to the armpit, ending in a crescent shape. It's commonly seen out of the holster in publicity/prepro shots.
The esb sidearm is a little larger (the main body) with no stock and, so far as I've heard, is never seen out of the holster. It's assumed that it is the same or like to the bespin security pistols. It was (is this confirmed?) made from a spear gun. The only reference we have for it is some behind shots that show the pistol butt, and the nozzle sticking out of the holster.
Can you see which one it is? Or if the picture is in color, which version of Fett is it?
Phil