First of all, in that picture you need to keep your end of the deal up. He would be giving you a lovely contact, but your elbows are locked straight and your hands are resting on his withers. Shorten your reins and carry your hands.
It also looks like your noseband is too low and too loose to do anything effective. And instead of tucking your bit ends under the noseband, you should buy small keepers for it.
As for bitting, I usually like to stick to a double link snaffle for all my horses, they eliminate any pressure that would be put on the pallate. You can get them in different thicknesses, from 14mm up to as thick as you can find is "legal" by the FEI rules. The thinner the bit the more harsh it is and the thicker, the softer it is. But too thick can put too much pressure on the bars and the horse will go with it's mouth open. A french link also works, but beware of Dr Bristols. The flat biscuit piece in the middle is actually very harsh and can put alot of pressure on a horses pallate.
The cheekpieces also have an effect. A Dee or a fullcheek are to help with steering and do not pull through the horse's mouth. So if you were to need to do a one rein pulley stop on a horse like an ottb these are ideal bits because they will pull the horse's face sideways without the bit going through the mouth. Dees are generally safer than Full cheecks though because it is hard to catch them on anything.
Another thing you might want to try for this horse is the Bee ring snaffle.
Be careful to put the smaller ring ontop and not on the bottom as the latter is improper use of the bit and will get you eliminated. This bit has some leverage, but a higher likelihood of being pulled through the mouth in the event of an emergency. It is also "legal" in competitions.
In dressage comps you are limited to snaffles. In jumping comps you can have leverage bits and in that case I would suggest a mullen mouth or large ported pelham with two reins if you know how to ride with one because you can apply a lot of leverage by lifting your hands up, and none at all when using it normally.
So long story short, with some adjustments to your bridle and purchase of full cheek keepers, as long as your horse is not sensitive to pallate pressure your bit choice is fine :)
Good luck at your show and always be ready to circle him if he gets going too fast.