It's an image that I would love to have imprinted on my remembrance cards when it is my time to leave this earth. Now, I realize that Fra Angelico's fresco depicts Christ freeing the souls in Limbo. However, I would like to think that the same principle would hold true for the souls in purgatory, once they have been set free. There is hope in this particular work of art, though. Like the souls of the just who were awaiting the Messiah were finally set free through Jesus' salviffic act, so, too, are the holy souls in purgatory awaiting that joyful day. There is precedence for praying for the souls of the faithful departed in Scripture. The reading from the Second Book of Maccabbees tells us that:
But the most valiant Judas exhorted the people to keep themselves from sin, forasmuch as they saw before their eyes what had happened, because of the sins of those that were slain.  And making a gathering, he sent twelve thousand drachms of silver to Jerusalem for sacrifice to be offered for the sins of the dead, thinking well and religiously concerning the resurrection,  (For if he had not hoped that they that were slain should rise again, it would have seemed superfluous and vain to pray for the dead,)  And because he considered that they who had fallen asleep with godliness, had great grace laid up for them.  It is therefore a holy and wholesome thought to pray for the dead, that they may be loosed from sins
May the souls of the faithful departed, through the mercy of God, rest in peace. Amen.