I'm still dreaming of an image file format where the file begins with the, say, 75x75px image, and then continues with higher and higher resolutions, where the

I'm still dreaming of an image file format where the file begins with the, say, 75x75px image, and then continues with higher and higher resolutions, where the browser can decide when to stop downloading. Ideally they wouldn't be redundant either, e.g. the 2nd block would contain just the data to upscale the 1st block to twice the resolution.. so the "75x75" thumbnail in a hidpi display would just need blocks 1 and 2..