Since you mentioned it, I thought it was kind of odd how the photo system worked here. On my screen the post images or “thumbnails” are bigger than the ones in the gallery view - or whatever its called when you click on a photo.
As @SpokanePeach mentioned, you could definitely cut down on costs by down-scaling everything to something like 1920 pixels on the longest side at upload. But yeah having small thumbnails that load instead of the full images is a good idea too. I think this would mean that you would need to generate a thumbnail image for every full size image uploaded.