Image / file attachment support?

[complexpoint]

Marked 2 preview of Bike files containing Markdown-style ![label](imageURL) entries

Import images

Another approach (avoiding subscriptions and an additional layer of proprietary url-scheme) is just to use Markdown links preceded by a bang, in the  pattern. Viewing a Bike file in marked2app.com will already display inline images included in the form of such links. I personally harvest the Markdown link using this Keyboard Maestro macro, which automates it for a number of macOS applications, including the Finder: