You can implement Firebase Authentication Anonymous auth, which according to Firebase
Use Firebase features that require authentication without requiring users to log in first by creating temporary anonymous accounts. If the user later wants to register, you can upgrade the anonymous account to a regular account so that the user can continue to work where they left off.
This should give you a unique download User UIDfor each anonymous account, see the farthest right column below the "User Auth" picture.
Edit:
Looks like I missed the second half of your question.
image_upload JSON " ", User UID URL- Storage Location. . , .
User Auth

