See the "Publishing" section here: http://developers.facebook.com/docs/reference/api/photo
Requires the publish_stream
permission.
To publish a photo, issue a POST
request with the photo file attachment
as multipart/form-data
.
You can publish an individual photo to
a user profile with a POST to
http://graph.facebook.com/PROFILE_ID/photos
We automatically create an album for
your application if it does not
already exist. All photos from your
application will be published to the
same automatically created album.
You can publish a photo to a specific,
existing photo album with a POST to
http://graph.facebook.com/ALBUM_ID/photos
.
The last sentence states you can publish to an existing album, so if you're trying to update the user's profile picture (not sure from the way you stated your question), try getting the album ID for the user, then publishing to that.