There are two images that stand out for me in Episode Four - Wanda Ventham looking absolutely divine in gold paint (and looking somewhat less inspiring with fake eyes painted on her eyelids - why did the makers of the show think they could get away with this?), and the body of Stael lying on the floor after having shot himself.
The suicide of Stael is shocking enough for Doctor Who. There have been characters who have sacrificed themselves to save others, but very rarely has someone taken their own life without having it mean something. What's even more alarming is that The Doctor gives Stael the gun! Not only do we have a suicide in Doctor Who, but an assisted suicide on the part of The Doctor. One assumes that The Doctor could see no way out for Stael (he does say to Stael that it's too late for him once he's looked into the Fendahl's/sexy gold Thea's eyes), but to see The Doctor tiptoe his way across the room, casually grab the revolver from the mantle, and drop it into Stael's waiting hands is somewhat disturbing. Tom Baker plays it well, though, not once looking at Stael while giving him the gun, and Stael's polite "thank you" once he receives the pistol is quite sad. Here is a man who was driven mad for power, then driven equally mad whilst betrayed, but still had the logical foresight to off himself and end his torment.
Overall, Image of the Fendahl is one of the moodiest and gloomiest of Doctor Who stories ever, and the absolute last holdover from the Philip Hinchcliffe/Robert Holmes era of Gothic horror. Things would get considerably lighter and more fantastical from here on in.