Blender as an image editor software? As a 3D/2D hobbyist myself who try to use the most free/open source software when I can, blender has become my default 3D software. For the 2D side (texturing and image editing) GIMP was the default one for many years but I start to reconsider it. It may be just me but I think most people start to change their work-flow from a destructive one to a non destructive one. In that regard, GIMP work-flow is mostly destructive. For example, if you apply a color curve tool in GIMP, its effects are "merged" with the layer and you lose the ability to change the curve later on. So I wonder, could Blender be used as an image manipulator in an efficient way or does the compositing in blender lack features for being a great (non-destructive) image manipulation tool ? Then, what free/open source software would you recommend for this task ?

I realize that the question is a bit broader that blender itself but I think it is fair to say that in order to do anything related to 3D, you will need, sooner or later, a good image manipulation software.
I'd say it depends on what you want.

For something like photo-manipulation (by that I mean color grading, adding effects like vignetting, etc.), the compositor can work very well (and non-destructively). After all, it's almost what the compositor was designed for. The only major difference is that the images were rendered with a camera, not Cycles or some other render engine.



However for creating images from scratch with 2D techniques like painting, blender is probably almost certainly not the best around. And using blender's image editor it won't be non-destructive (though for something like this I'm not sure if a non-destructive workflow makes a lot of sense?).

Painting is not my strong suit, but there have been some pretty amazing things done with blender's painting tools. Also note that blender's painting tools recently got a nice upgrade as part of the GSoC.

That said, if painting is what you are looking for, you might be better off with a program designed for painting, like Krita or MyPaint.