Personalized field

Hi! My blog’s theme, Daily misery, as you can see here https://www.thedecline.net have a left image on every post. I want to know how to put the same image to every posts of the same author. You know, in my blog there’s three authors so I want to put an image on all my posts, so people who read it can know who wrote it.
I hope you can understand me, my english isn’t perfect.