Would it be possible to increase my opinion, if I protect my liege or earn prestige in a duel for my liege? There should be events, if you are the commander of your liege and are assigned in the same battle. Especially, if your son got wounded and you try to protect him. There are many possibilities with this new mechanic.
Your liege (and others) can be impressed with your actions on the battlefield, yes. Aside from that, characters will in general look favorably upon (have increased opinion of) experienced duelers.
Does this mean all current portraits will receive improvements?
Any character who gets a scar or a black eye will have the appropriate layers added to their portrait. As for the tattoos and warpaint, they are tied to Warrior Lodges. Then there are the other general improvements, like Harelip, or the Fat and Malnourished traits, which have been presented in earlier dev diaries.