Using digital certificates in digital signatures

Please could anybody explain to me how certificates are used with digital signatures?

I understand difference between certificates and signatures. It is clear.

Digital certificates are used to verify the trustworthiness of a website, while digital signatures are used to verify the trustworthiness of information.

I am not sure if i understand this picture on wikipedia.

Bob generates private key and public key. He encrypts hash of message with his private key - signature. Bob sends message with the signature and public key to Alice. Alice generates hash of message and compare this hash to decrypted signature. If she wants to decrypt the signature, she needs to verify the public key by certificate (if this public key belongs to Bob).

Why is certificate next to Bobs signature? I thought certificates are used with only public keys.

Is public key always used for decrypting and private key for encrypting in digital signatures?