Krita [1], probably. They focus on painting nowadays because they don't want to be competing directly with GIMP, but it's a very capable photo editing tool. I use Krita for a lot of my photo stuff because it supports more than 8 bits per channel. Still, I think GIMP has enough developer attention, and GEGL is powerful and compelling enough, that GIMP won't get replaced any time soon.

[1] https://krita.org/