It's really hard to give advice without seeing you lift in the gym.

In general I believe a low-incline dumbell press is probably the best movement for Chest and if I was limited to 1 exercise I would chose it. That being said, I have seen some guys have trouble getting a nice arch and keeping the Chest high on this movement so for them I would recommend trying a bit more incline or flat dumbells and see where your form is best and where your Chest feels the most engaged.

Above is my general recommendation for bringing up a weak Chest. After looking over your pics and your description, it seems like building up the lower chest area might help achieve the look that you want. Medium grip Decline Bar Bench to the nipple have a way of putting on a lot of mass quickly to the lower pecs, for most guy this isn't necessary but in your case it would help to make your Chest bigger quickly and fill in a lacking area.

I would do both movements twice per week if possible in your split and focus on really feeling the Chest work 8-12 reps and you will definitely have to lower your weight a bit to be able to stop the shoulders from taking over.

Also like UBC said above you can't bring up a weak point too much during contest prep but it never hurts to start training the muscle better.