It is risky to sign screenshots and send them to others. Pictures with your autograph can be put in the signature column of any contract, which has great potential risks to your personal interests. Signature and seal have legal effect. In actual civil activities, there are usually other evidences to prove it. Important civil activities are generally confirmed by seals, notarization and other means. In criminal or some civil activities, fingerprints are often added to prove that they are the same as signatures. In short, it is not a casual signature that has legal effect, but a signature is a kind of evidence and voucher to prove the expression of meaning. Generally speaking, this will do. If it is not enough, there are many other means.