If you update or modify an image element in the Mobile App, there may be some time for users that already have the application to view the updated image element.
The reason for this time lag is because the server must distribute these images across all the various apps. Each individual app will check for updates in their own time, based on a variety of factors. So, one app may receive the image change sooner than another.
Anyone who downloads the app after you change the image element will see the modified image immediately upon download. This is because the app would be the most recent version.