I see a couple of indicators of instability and a couple of anatomical variants which further add to the problem.
First the indicators of instability:-
C. Looks to me like the posterior longitudinal ligament has stripped away from the back of C2.
This ligament is the black line that grips all the vertebrae and holds them firm.
There should be nothing between that black line and the bone.
D. Looks to me like the posterior arch of C1 is jutting further than it should into the spinal canal.
Second the anatomical variants:-
A. You may have a short clivus (the bone to the front of the opening in your skull)
B. You have a retroflexed odontoid
Do a search in the box above for further information on each of these two variants.