You could either leave the origin in the centre and have it snap to a 16x16 grid
or set it's position alongside rotating it like:
set angle to 90
set x to self.x+self.width
but of course a different position would be needed for each angle.
If you need them to rotate more than once, so it works regardless of their current angle, you could have an imagepoint in each corner.
So for rotating 90 degrees:
set position to self.imagepoint(TopRight)
set angle to self.angle+90
and for rotating 180 degrees
set position to self.imagepoint(BottomRight)
set angle to self.angle+180
ps: this should have been posted in the 'how do i' forum