In the left hand corner of this picture is depicted the souls in Purgatory. (Note the angel in blue, who is pulling a soul out of Purgatory to take it to heaven). This painting is in a chapel in Rome, Italy.
Our Lord dictated the following prayer to St Gertrude the Great to release many souls from Purgatory each time it is said.
Prayer of St. Gertrude the Great
Eternal Father, I offer Thee the most Precious Blood of Thy Divine Son, Jesus, in union with the Masses said throughout the world today, for all the holy souls in Purgatory, for sinners everywhere, for sinners in the universal Church, those in my own home and within my family. Amen.