A question: Should the HiDPI setting be respected also when Link to originals is chosen?
An interesting question. At present, the jAlbum core ignores the HiDPI setting if you choose Link to originals, since it's not generating any images - it's just using your original, untouched.
But an argument could be made that it should do with originals exactly what it does with scaled images, i.e., lie to the skin about the imageWidth and imageHeight. In other words, if you supply an original that's 4000x3000, it should tell the skin that the dimensions are 2000x1500. So, on a normal monitor, you'd see a 2000x1500 image. On a Retina display, it would display as 2000x1500 CSS pixels, but the display would be using the full 4000x3000 image to fill that 2000x1500 space, thereby yielding a sharper image.
ETA: The problem is that some skins would really choke on this, particularly those that are using a third-party lightbox script, one that can't be altered easily.