I do most of my graphic work in Paint.net for simple stuff (it's free) or Photoshop for complex work (it's very not free). Both on my laptop.
There are graphics apps for Android, but it's a lot easier working on a larger screen. (And in Paint.net, changing the size of a pic takes less time than saving the file when you're done. Just 2 clicks, type in the new size [in pixels if you like, or in inches or cm] and save.)