If it is image editing nothing can beat Photoshop Express. Photoshop is being developed day to day. It's such a powerful app and being more powerful with Imageready. Corel draw and Adobe illustrator are also be considered when you are into making your own drawings to your apps.
Nothing beats the Photoshop. And they also have the cloud version of the game. You can also make use the web based image editors like Picmonkey and Crello along with the Canva. That should give you an idea on how to edit the images and design the ouput which you can export.
Adobe Photoshop and Adobe Lightroom are considered the best image editing software for mobile app development due to their powerful features and flexibility.
Photoshop is being developed day to day. It's such a powerful app and being more powerful with Imageready. Corel draw and Adobe illustrator are also be considered when you are into making your own drawings to your apps.