What I want is simple, but had no luck finding that so far:
A canvas.
I can add raster images to it. (Does not matter how, but preferably all 3 actions: "file->open", drag&drop, paste from clipboard.)
They stay as "objects" that can be freely moved on the canvas, WYSIWYG-style.
They snap to their edges.
I can export the final assembly as a raster again. With 1:1 fidelity of course.
A very rough approximation could be probably thrown together in a few days, but...
Do you know of anything like that?
Paint.NET can do this. If you want the objects to stay movable I would paste them as layers, so you can still move them.