@jem – I am in the same use scenario situation as you, having adopted MD for all its advantages, while at the same time working with images as vital part of my note/knowledge/document setup.
Actually, I think you are representative of a larger cohort of people coming to work with text documents alongside photos/images in the current contexts of standard and global documents formats used nowadays.
This is also, I would think, why the request to include Textbundle format natively withiin DT as document manager is quite frequent around here. A search for it immediately surfaces more than half a dozen explicit requests, while this also regularly comes up as wish within wider context of discussions here.
The good news: DT team seems to have it on its radar, as one of the more recent threads on this topic documents.
And, as @jonmoore correctly pointed out in this thread, while Textbundle is per se not a mainstream format, it has a firm strategic place in exactly this use-scenario you are addressing: modern markdown text and images in multi-app contexts
– … as it is useful & critical especially in terms of requirements of *interoperability*. This is of course because – as you & others here will surely be aware – the combination/pairing of MD-text and image-links/-embeds is notoriously tough to manage from the getgo ( … outside of DEVONthink, that is 
).
For that reason it seems indicative that venerable Mac stapleware like Bear, Ulysses, Craft, Marked 2 do support it.
Let´s hope it find its way into DEVONthink!