The fundamental problem in my mind is the mixing of binary and text content. An optimal solution would separate them, allowing systems like Git do the versioning. But separating the tightly coupled parts into own files would also be annoying sharing/management wise.
Base64:ing the images into strings, like one could do with html, would probably not be ideal for compression. As a matter of fact, text-files as such would not be ideal compression-wise.
So I suppose if binary-format cant be avoided, SQLite would be as good as any other compression format. But without built-in collaboration protocol support, like CRDT, with history truncation (and diverged histories can always fall back to diff) I dont think it'd be good enough to justify the migration.