2306 started out with this as one of the proposed solutions, but has changed direction over time...

None of the layouts that we ship with core has node/% as path, thus people trying to add a field as a block won't be able to figure it out unless they have good knowledge of the internals of layout paths/contexts. This is something that has made me scratch my head more than once and it honestly is a shame to have in-built such an amazing feature, but not expose it prominently or at least in a way that's easy to figure out.

We could ship core with another layout that has its path set to node/% and includes a field block. A good candidate would be a "Post" layout, with a sidebar region, with a visibility condition set for the layout to apply only to post nodes. The main content would be a display without the image field, so that we can have that as a field block placed in the sidebar region.