Emacs supports inline graphics; you don't see the capability used all that often, but it certainly exists, and has done for quite some time.
Inline documentation doesn't seem all that desirable in the first place; from the screenshots I've seen, it looks like it would get in the way, which documentation in a separate buffer doesn't do. (I'm half tempted to implement inline documentation for Emacs Lisp mode as a proof of concept, mind you, but I really don't see the point of it.)
Previously that had been one of the differentiators between GNU Emacs and XEmacs, the latter more enthusiastically adopting GUI-, image-, and mouse-related features. I believe XEmacs got inline-image support sometime in the mid-'90s, and showed it off by including a display of the XEmacs logo in the default startup buffer.
Inline documentation doesn't seem all that desirable in the first place; from the screenshots I've seen, it looks like it would get in the way, which documentation in a separate buffer doesn't do. (I'm half tempted to implement inline documentation for Emacs Lisp mode as a proof of concept, mind you, but I really don't see the point of it.)