You can change the center of rotation on an image by using setCenterOfRotation.
As for the top of the turret, it depends on what your goal is. If, for example, you wanted to render another image or particle system at the head of the gun, you would be better off rendering the two elements in place (i.e. initial angle), and then making it rotate by calling g.rotate before and after drawing the various turret parts.
On the other hand, if you are only trying to find the rotated point for the sake of shooting a bullet, you might find it easier to use vectors for bullet movement. See here:http://www.rodedev.com/tutorials/gamephysics/
Lastly... if you really need to find the rotated point, you could look into 2D rotation:http://www.siggraph.org/education/mater ... 2drota.htm