You should get some image editing programs and see which suits you best. I hated Paint.NET because I had no idea how to even make a blank canvas and draw a single dot on it.
Photofiltre isnt userfriendly either. It expects you to know a lot about it before you use it. For example you can't just start drawing on a canvas right away. You'll need the right pencil, another layer, the right pencil size etc etc etc.
I've never tried Gimp but mainly because I hate having 'demo versions' of software. It's like you get spammed with pop-ups like "HEY WANNA DOWNLOAD THIS? GET A 2% DISCOUNT NOW!" and shit so better get yourself something free.
Photofiltre is free, not talking about Photofiltre Studio X (which you have to buy / get a code for). It's easy to use in my opinion because the taskbar is on the RIGHT and it's obvious what the icons do. Effects are easy to place in, but it's not needed for 2d top-down sprites. Perfect program for me.