I have been using that method when I have needed to get graphics from the gam. I have had both Dominions and Paint open, tried to zoom the lone attacker to about the same size as his spr1 open in the paint, then screencaptured etc.
I normally get a test game just for catching that sprite, and sometimes I make a quick mod that adds that unit type a ranged weapon like Mind Burn. That makes it much easier to catch him alone.
You can also (as I do) make your own attack .tga files. A good way to this is using the option called 'rotate the layer or selection' in gimp. Just mark eg. the arm and rotate it. Sure you need to patch it and make some minor adjustment afterwards, but the result is rewarding