A picture/image of the signature will be an image of the signature.
Anyone can then use it, period. In a real sense, that signature would
have no legal value as it's source is not verifiable. It would not be an
"official signature", merely a "courtesy signature". As long as they
understand that there isn't any protection that can prevent its reuse,
and it has no legal standing, let them do as they wish. Think on this,
if someone has a document(paper or image)with your signature on it, they
can scan it and crop it to the signature, and you can't do a thing about
it. THAT is why it has no value.