Im unsure how to let users upload a picture from there library onto my app.
You can use the file from device block.
Not sure if you had this issue already - but make sure you follow the strict policies on uploading files/pictures into a public app.
Both Apple and Android gave me headaches on how this is should be done, for example including a “flag” button where users can flag an inappropriate content or else.
Even if the block itself is quite simple to use, the application is not as simple as it seems due to stores’ policies.
Good luck and hope it helps!