I think the main reason is that thumbnails are just not needed these days. Networks are getting faster and more reliable these days and you have already proved that there are alternatives to loading full size images. So why is there the need for adding thumbnails to large images – since you’re going to be downloading these images anyway? Thumbnails have their uses, but I don’t think they should be included in the bigger picture. If you need to have them, then they should be a separate image and loaded appropriately. I think that having them as part of the main image is for the lazy. If you need them, then code them. If you don’t, then they’re not needed anywhere.