Pictures to local Storage for Canvas

I want to save some pictures to my local store, to use the application offline.
But when I open my app in AirPlane mode, the background Picture of my Canva doesn’t charge, it’s a white square.
And Is it possible to change the background picture of Canva by block? The user chooses a file by file picker. Or do I need to add a picture object behind Canva, how can I do this to have the same proportion?