Can I legally show pics of my husband and mistress to anyone I want?

The answer depends on how the pictures were obtained and what the pictures depict your husband or his mistress doing.  Generally, photographs taken in a situation where there is not an expectation of privacy are fair game.  However, if the pictures were obtained in a manner where you husband and anyone else in the picture had an expectation of privacy or the pictures depict certain personal acts, your husband and his mistress could have a cause of action against you, depending on how you showed them and who you showed them too.