There's a huge difference between video and images, video is always expensive bandwidth-wise and intense. I can definitely see it having an emissions impact. Nearly all the images you'll encounter on your day to day browsing otoh is tiny and heavily compressed, bigger than text, but not enough to have a notable impact like video can.
As far as reverse tethering, it's under USB "internet" in settings, once that's going you go back to network connections in Windows and you'll have the computers Ethernet and another Ethernet that your phone is emulating. Select your computer Ethernet connection, right click properties, sharing tab and share to the other to bridge the connections