Dear JohnDoe,
When I was a younger guy, and had what I thought was a great idea to do something, I always hated it when someone would spoil it with: “but it already exists”.
But here I go. Sorry. But it already exists.
I agree that GIMP is much too bloated for many jobs. But what about gthumb, photoflare, darktable, shotwell viewer, pinta, or photoxx, or showphoto, or at least take one of these and fork it and make it into what you’re looking for?
Also there is ImageMagic which has lots of solid support for image operations and types of formats. I read somewhere the most of the above suggested image programs actually use ImageMagic underneath do do the work.
Thanks for your energy.!