A useful primitive would be "getDevicePixelRatio". And possibly a "setDevicePixelRatio". This would let the image control if it wants normal-sized pixels (2x2 on a retina display) or actual device pixels (which would make everything way too small with an old image).
This is how HTML exposes "retina pixels", which seems like a useful abstraction. You could look there for the spec (I think devicePixelRatio is 2 on retina, 1 otherwise).
- Bert -