Hy, thanks for suggestions.
1: there already is an import option, for .aco .gpl and .png files (the little arrow on the bottom-right of color picker), if you think another type is missing, I will add it.
2: It's an hidden feature (because it isn't stable for now): right-click on pipette to activate and pipette will work with the image behind too.
3: In my opinion, it's out of the scope of this tool; I know it's hard but rotoscoping is handmade paint and have better result because you do choices...