Currently, TinyMCE can't deal correctly with image URLs in drafts. A relative URL, which would be correct after publication, can't be resolved in the editor.
The issue is that TinyMCE resolves links relative to the current URL, which is the drafts folder. However, the entry attachments live in a subdirectory of the drafts folder, so the URL that TinyMCE uses isn't valid.
Currently, TinyMCE can't deal correctly with image URLs in drafts. A relative URL, which would be correct after publication, can't be resolved in the editor.
The issue is that TinyMCE resolves links relative to the current URL, which is the drafts folder. However, the entry attachments live in a subdirectory of the drafts folder, so the URL that TinyMCE uses isn't valid.