Sometimes the right tool is another app. MojoPad cooperates instead of competing.
Edit ▸ Edit Page in External Editor opens the current page in your text
editor — Markdown pages as .md, plain text as .txt, rich text
as .html. Every time you save there, the page updates here, live. Pick the
editor in Settings ▸ Editing (leave it blank for the system default).
Double-click any image in a page and it opens in your image editor — Acorn, Pixelmator, Preview, whatever you set. Crop it, mark it up, save — the image in your page updates the moment you do. Option-double-click saves the image out to a file instead. (Sketches are the exception: double-clicking a sketch reopens MojoPad's own canvas — see Formatting.)