You're able to change the default images for your mobile app under Images section as showin in the following guide.
Uploading Your Own Images
Head over to
Images section in Saturn:
Choose file button on the image that you want to change. Your image size must be
660 x 440 pixels.
By clicking the
magnifying glass icon, you can see a screenshot of where the image is placed in mobile app.
Once done, click on the