I second this request. For now I will make do with 2 separate gallery folders (1 for 72dpi,  the other for 140dpi images, both sets with images of 1024p wide x 768p high).

Since bandwidth is increasing in leaps and bounds, and mobile usage set to eclipse desktop usage by 2016, I'm hoping this is a temporary headache, and that the day will come when I can just use one hi-res gallery for all devices, and ye olde lo-res phones, etc will load these in a blink versus the current choke-and-hang experience. This doesn't negate the need for a more elegant solution than the duplicate galleries+js workaround suggested above.