I'll second Aseprite as it is mostly intuitive and has some great features. I particularly like the FX options. On my iPad I use Pixel Studio.
I honestly haven't done any original work in aseprite, mostly I use it to edit pre-made graphics -- stretch, resize, distort, etc. In a lot of games it's essential to have images with a partially transparent background and I don't think MS paint supports that.
The guy on itch.io that first recommended Aseprite to me is using it on his games. I've been pretty impressed with his animations.