There is currently no atlas/texture selection mechanism based on retina/low res. The easiest path now is to use a large texture and scale it down. Make sure you tick the “high_dpi” setting in the game project and the engine will create a hi-res buffer where the graphics are drawn on devices that support it.
If you want to create different versions depending on screen size you will currently need to set that up with some off-line build pipeline. We have a ticket in our backlog to start working on a design and eventually implementation of a high_dpi pipeline solution, but that’s in the future somewhere.