The bandwidth used for single images is dwarfed... massively by streaming video content... I don't think it's a use case that is worthy of excessive time/effort to optimize for size because of bandwidth cost/effects.

edit: I'm not saying don't do it if you can get away with a smaller image dimension, or do basic optimizations... but if your concern is the environmental impact from a simgle high-resolution image from a photography site, you're putting the emphasis in the wrong place.