Being able to fetch different resolution images depending on the device is good for all the reasons UIKit natively supports @#x, plus saving bandwidth. Something like srcset seems roughly right, and the code could look like:

<Image
  sourceSet={{
    2: {uri: 'http://example.com/image.php?w=200&h=200'}
    3: {uri: 'http://example.com/image.php?w=300&h=300'} 
  }}
/>

cc @vjeux you've probably thought about this