On devices that run Android 4.4 (API level 19) and higher, your app
can interact with a documents
provider,
including external storage volumes and cloud-based storage, using the
Storage Access Framework. This framework allows users to interact with
a system picker to choose a documents provider and select specific
documents and other files for your app to open.
Because the user is involved in selecting the files or directories that your app can access, this mechanism doesn't require any system
permissions[READ,WRITE Permission], and user control and privacy is
enhanced. Additionally, these files, which are stored outside of an
app-specific directory and outside of the media store, remain on the
device after your app is uninstalled.
Note: If your app uses the media store, however, you must request the READ_EXTERNAL_STORAGE permission to access other apps' media files.
In the above case (image picker), uses ACTION_GET_CONTENT
to open an file , which opens a system document picker using SAF framework and doesn't need permission.
Talkin about Data protection - When opening file using ACTION_GET_CONTENT
- Because the user is involved in selecting the files or directories that your app can access. App has only access to a URI of image selected by the user, and there is no way to automate the image selection process, so user image is not altered in any way possible, and remain on the device after your app is uninstalled.
Image picker copies the result URI from document picker into a temp cache of your app, from where the image is uploaded to a firebase(or any other place you need a image) - See this answer to see how it picks the image - Flutter image picker.
Since this a official way provided by google - explained here, app should not cause any security threat to the user privacy
Further if you want(I hardly feel that needed), you can show a permission declaration dialog , stating that app is going to navigate to a file picker and saves a temp copy in app cache, not altering the original image in any way(or something similar you fells right).