How to get a picture on my profile?

Unfortunately you cannot change or add a profile picture at the moment.

Yahoo are in the process of developing the profile and account and have removed this facility along with nicknames and avatar changes for the present (ongoing since January). Nobody knows when we are going to get this ability back (if ever).

Whatever you do, don't telephone the numbers given purporting to be yahoo assistance. They are scammers and nothing to do with yahoo. They charge exorbitant fees for their services whilst genuine yahoo assistance via email is free.
Please report them as spam

Avatars and the editing of nicknames have been disabled. They are making changes to profiles. No one knows when or if they will be enabled again.