Save 3d plot to an image

edit

With a Jmol-produced 3d plot, saving to an image results in different output to that which is displayed on the screen.

I have

G.show(aspect_ratio=1)

and that looks lovely, but

G.save('tmp.png',aspect_ratio=1)

produces something quite different, I suspect because G.show() uses Jmol, and G.save() uses Tachyon. The "export" menu item of Jmol doesn't seem to produce very good results either, as the image is not properly centered on the page, and sometimes is even clipped. Any ideas how I can go from my Jmol image to a nice image file? (Other than taking a screenshot!)