Since Markdown is now the web's rich text, its image tag is probably the thing to try. Then, Emacs is guaranteed to be able to show images. Possibly Intellij Idea, since their MPS supports wacky stuff in DSLs' editing interfaces. Dunno about Sublime etc., but I hope Atom and company can make proper use of their platform. Even if not inline, an image can often be opened in a split frame.

BTW, Mermaid and such might be more suitable for embedding diagrams in comments―maybe even just one of the ascii art diagram languages (for which there are utils to convert to graphics).