The problem with most smartphone image editors is that they are meant for quick edits to photos. Usually they are limited to cropping, adding filters and drawing. I’m looking for something a little more capable, something more akin to a traditional desktop editor like GIMP. Open source is a plus.
The best one imo is Iudesk Photo Editor. Not good for drawing, but otherwise it has almost everything, ad-free version is just a few €$£ and minimum tracking compared to the big guns.
Open source options are lacking: Litrato, Pocket Paint and Simple Draw, each does something but still not much together.