We released a version of GIMP for Android. It is really the full Linux edition, but running on your phone or tablet. It is FOSS and the source code can be found here: https://github.com/CypherpunkArmory/GIMP
If you are not familiar with GIMP. GIMP is the GNU Image Manipulation Program. It is not a Photoshop clone, but it is similar in that it is a powerful tool for photo retouching or image editing and authoring tool. https://www.gimp.org/