My thought on the ending is that it leaves Bull an opportunity to come back......possibly to another network/cable channel. Perhaps he and Marissa figured out a way that he goes away now so their client goes to jail, and his way of indicating he is coming back is to leave the photo. If they moved TAC to another state, he could possibly continue in the same profession or some ancillary business.
I always thought the glasses were a prop anyway. Something he didn't really need, but it made him look more intellectual perhaps, or showed him as a serious-looking person for the jury. Don't know....just that I never thought he needed them for seeing purposes.
Hopefully, the director/producer will enlighten us sometime.