We haven't noticed any kind of issues with portraits themselves.
Portraits are flickering how exactly?
Does it happen all the time, or only in some specific situations/places?
What OS do you use?
If you tried re-installing the game, did you also remember to delete the "Crusader Kings II" folder at C:\Users\USERNAME\Documents\Paradox Interactive\Crusader Kings II? It's usually a good idea to do that as well, as to install the game from scratch. Just keep your save games if you have any you want to keep playing before you do delete the folder!
Portraits are flickering how exactly?
Does it happen all the time, or only in some specific situations/places?
What OS do you use?
If you tried re-installing the game, did you also remember to delete the "Crusader Kings II" folder at C:\Users\USERNAME\Documents\Paradox Interactive\Crusader Kings II? It's usually a good idea to do that as well, as to install the game from scratch. Just keep your save games if you have any you want to keep playing before you do delete the folder!