Yes, that was it, I thought you need to map new textures to plane so that they follow the curvature, for this you would need texture coords based mapping.
Regarding getting a bit darker image in GIMP, check that you use 32bit float project. 8- or 16bit integer projects can introduce precision issues and can also clip your data.
MartinZ, in PS you would have to probably build a plugin, I’m not sure there is a tool to achieve it. Might be though, I don’t know all the ins and outs of PS.