Image data is just binary blobs. You aren't splitting that into channel columns or anything. But an image file for an editor like Gimp isn't one image blob. It's dozens or hundreds of them - one or more per layer - along with tons of associated metadata at every level.

All that tends to fit sensible schemas and managing it is what SQLite shines at.

I don’t see how this addresses my point that zipped XML gives you the same thing, but simpler. I understand that a GIMP file is many images, so that makes a zip feel like a great fit to me. The only advantage I see for using a full-blown DB is ensuring consistency with references, which admittedly is a plus. But beyond that, what do you gain?