Well I don't know much about the flash you're using but normally the trigger is a small pulse applied to a small high voltage transformer which creates a few kV to ignite the tube.
The trigger pulse is normally a couple of hundred volts but low current, which is why a high voltage transistor is needed. It's also often delivered by a capacitor so a TRIAC might work.
The circuit you posted should trigger the flash just fine the way it is. The Diode portion of the optocoupler has a forward voltage of 1.1 volts at about 80 mA. Therefore if you want to trigger using a 5 volt pulse you have 5 volts (supply) minus 1.1 volts (forward voltage of the diode) gives you 3.9 volts. Therefore 3.9 / .080 = 48.75 and thus your 50 Ohm current limiting resistor. Connected just like the image the flash should fire when you apply 5 volts to your opto coupler with that 50 Ohms in there to limit the current.
Sorry if I mislead you, there's no need to buy a optp-TRIAC, if you already have a photo-transistor, however if you can't get hold of a photo-transistor go with the opto-TRIAC.