"Above his head they put the charge against him, which read: “This is Jesus, the king of the Jews.” ~ Matthew 27:37

The painters who painted the Crucifixion worked more than a thousand years after the fact. Therefore, they each had a substantial amount of artistic freedom in expressing the scene. At that time (the Middle Ages), all (Roman Catholic) Church Masses were conducted in Latin, and nearly everything associated with Christianity was written and spoken in Latin. VERY few books existed (before the printing press was invented) and only the Church and government normally had them, so virtually no people in a Congregation had any books, including the Bible. However, a number of Christian phrases WERE commonly known by the people, and one was Jesus' official crime, commonly referred to by four Latin words, IESVS NAZARENVS REX IVDAEORVM

Since anyone who would actually see the paintings of the Crucifixion in the Middle Ages (inside a Church) would then already be familiar with the phrase (in Latin), the painters all apparently chose to simplify their paintings by only including the initials of the four Latin words, I N R I. The early Church had also apparently used that abbreviation.

The translation is "Jesus the Nazarene, the King of the Jews"

Happy Easter Everyone!