In Obsidian, the file in photos is copied into the Obsidian vault when you select it. Hence it can use a relative reference to the file at that point.
Drafts could easily insert the text content for the image reference, and send that over to Obsidian. The tricky bit however is referencing an image held in the photos app.
As a text-based tool, Drafts does have the capability to do some work with files, but primarily text files, and nothing for interacting with files held within the Apple Photos application - they are not available through the files app.
In a similar Shortcuts-based approach to the one used by @tf2, you could replicate the Obsidian process (not using external hosting), and you can integrate that with an action in Drafts that adds the image reference. But Drafts alone is not currently able to interact directly with Apple Photos.
Hope that helps.