Unless you go out of your way and convert the images to an array of numbers and include them in that way in your source code so that they become part of the built binary, your app is not a derived work of the images it shows. This means that the images and the app itself can be licensed independently.
As the original images are under the CC BY-SA-4.0 license, the cropped versions you created need to be under the same license.
The app itself can be under a completely different license, including a closed-source one.