the other photos doesn't show any lock, no, you don't have a lock
if you removed the two screws on the metallic shield of the gpu on the back of the case or if the case has a single big screw locking all expansion cards and you have removed it
the movement should be softly pull from the side where the monitor connectors to the back, but softly, if you can't do it and you are scared, well, let it in place
try to find some videos to understand how to remove, reassemble it just to discard a problem but a mainboard can be bent alot so i wouldn't worry unless you heard a "crack"