Editing to Add: Apparently, the reason Himself is so popular as a prompt is that his own style is actually a mish-mash of about a dozen other artists from the Classical era. So instead of a string of all those names in their prompts, AI users took the short cut of using "art by G***.R*******." in order to not have extra long style prompts that would just get ignored. Can't remember the article wherein it was mentioned but if I run across it again I'll post a link.