You can set the URL for a profile using https://docs.braincloudservers.com/api/capi/playerstate/updateuserpictureurl

Uploading an image is a separate issue. You could upload it using the User Files api - but be sure to set the shareable  flag to true.

To upload a file - https://docs.braincloudservers.com/api/capi/file/uploadfile

Note - instead of allowing users to upload their own files, most systems these days would allow them to choose from a set of files. You could have a bunch of avatar images available in global files - and set those URLs on your player via the UpdateUserPictureUrl() call.