A big thank you to the anonymous commenter who alerted me - here's the first promotional photo for The Big Four! It's amazing to see the 'Big Four' (Poirot, Hastings, Japp, Miss Lemon) back together again!
Source: http://www.cyfraplus.pl/ms_galeria/fotobase/41551_c.jpg. Copyright ITV.
UPDATE: Commenter "ueetba" just alerted me to two more photos (see below)!
Source: http://www.alekinoplus.pl/program/film/poirot-wielka-czworka_41551. Copyright ITV.