Hi Charles,
I recently stumbled on another such case from a Xiaomi phone user. Looks like Xiaomi, on some of their devices, defaults to their own Files app as the system file picker. Their Files app does not support a Dropbox integration, only Google Drive and Xiaomi cloud.
This is why you can't upload your images. I'm not really sure if you can change this behavior somewhere in Android but I suspect you cannot.
Martin