The painting you are referring to is likely "Saint Catherine of Siena" by the Italian Renaissance artist, *Caravaggio*. However, the specific imagery of "Saint Catherine touching the foreskin of Jesus Christ" refers to a different aspect of Christian iconography, particularly related to the cult of the Holy Prepuce, which was regarded in medieval times as a relic of Jesus' circumcision.
Saint Catherine of Siena is often depicted in various works