Hi,
I would create a new collection, call it, say, DBImages. Add property “Image” (to store the picture) and add a relation to User, one User can have multiple images.
Then, on the app screen, add an Image Picker and an “Upload” button. Add an Action for upload button - Create new DBImage, then select the Image Picker as an image source and current user as a User.
As a result you will get a relational database table - DBImages from which you could easily select Images belonging to a particular user (by creating List with filters).
One side note - seems that Image Picker does not support adding multiple images at a time, so user will have to upload them one by one.
Just my 2 cents )