You can set the URL for a profile using http://getbraincloud.com/apidocs/apiref/index.html#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 - http://getbraincloud.com/apidocs/apiref/index.html?csharp#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. 

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