Hi Richard,

eee? Do you mean the centre of rotation of background image or object? For rotating object the centre of rotation is set by picking first point when using Rotate command; or am I wrong? For rotating around line you can use Rotate-Axis command. You cannot use existing one, though.
I guess there is no currently way of changing centre of rotation of image. However, Michael was thinking about something for the frame of object -- see the last paragraph in this message.

Petr