> > This isn't just resampling an image
> GPUs can do that, too: http://fastcompression.com/products/jpeg/cuda-jpeg.htm
Agreed - but for how many different formats, and how well do those implementations support all of the various format options for things like bit depth or palettes, compression variants, etc.? That's not just things like compliance testing – itself a big problem – but also handling all of the slightly non-compliant data in the wild which users will inevitably expect to work.
(I'm somewhat biased having spent time dealing with JPEG 2000 imagery where various lapses on the standards side meant that it's still common to find images which don't display correctly in one or more implementations but are silently reported as correct in others)
Again, I'm not arguing that doing this on a GPU isn't a good idea — the hardware has become common enough that it's reasonable to assume availability for anyone who cares — but just that there's significant overhead cost for anyone who needs to handle images from unconstrained sources. It'll happen but this kind of thing always takes longer than it seems like it should.