[a/b]

I've loaded the AGA picture into piccon as 320x240x256col, and the green/red jacket guy is the last person on the right side, there is no red/blue shirt girl next to him, she is the first person on the left side of the image.
It looks like the actual data you load from disk is shifted, nothing wrong with code/copperlist.

edit:
Looking at your python script bitplanelib.py, the problem is that you have also included picture dimensions (4 bytes) at the start of AGA picture. That's causing the shift by 32 pixels.