Manually
Maybe it's possible to hardcode the location in your manifest after exporting the C3 project, if you can find this folder res/drawable/, then in the manifest you could put the image inside res/drawable/filename.png and try replacing in the manifest @drawable/banner with res/drawable/filename.png or @drawable/filename.png.
C3 Files
If you want to do it via C3, you can upload a file to Files in C3, it will be located directly in exported www\filename.png. You can try to reference that image from the exported folder by replacing @drawable/banner with www\filename.png, there's a possibility that it needs to be served via protocol like: http:\\www\filename.png
Or the syntax could be @drawable/www/filename.png. Their docs are kind of weird so I'm guessing a bit here.
https://developer.android.com/guide/topics/resources/drawable-resource#Bitmap
I hope this helps in any form further, otherwise, at least the post got bumped a bit :)