I'm not looking for file differences, I'm comparing visual diffs to see which image I prefer. Windows has some apps that support this, but I guess I'll just alt-tab between two separate image viewer instances
Discussion
If you use i3 or another tiling window manager you can do away with the alt tab.
perhaps you can use layers, to overlap them. and use the hide layer option.
Doesn't tick all the boxes, but still prob better than alt tabbing. https://beta.flathub.org/apps/org.gnome.gitlab.YaLTeR.Identity