how do the paintings compare in terms of their subject matter
The subject-matter of phonetics.
You
The subject matter is what the book is about. So if you have a book on, say, Winston Churchill, he is the subject matter.
Subject matter is what an art work has been created about. For example if you were to have a painting of a cow, then the subject matter is a cow.
The subject in the poem "Does It Matter" by Siegfried Sassoon is "you."
The learner learns the subject matter
A subject matter expert in the hazards or recovery activities underway within the incident site.
The subject matter is Dora Maar weeping.
Subject matter, theme, matter in hand, field, subject...
The subject matter is a dove of peace! It is a symbol for peace.
Iconography is the study of the subject matter of art.