The Scream will always be a favorite. It's just a wonderfully weird painting, but it's also simple and complex all at once, as you've touched on here.

Vermeer is a favorite of mine, if you haven't already done a writing on his work. I would suggest you review a documentary called Tim's Vermeer hosted by Penn of Penn & Teller fame. It puts an interesting perspective on Vermeer's body of work.

You might also consider doing write-ups centered around a particular technique which is no longer in wide use, like paintings done in egg tempura or chiaroscuro style. I find the technique history of art just as interesting as the thematic and social history of the discipline.