The titular altarpiece is probably the work of Bormla-born Rocco Buhagiar (1725-1805) although some attribute this painting to Francesco Zahra.
Can. Rev Joe Abela in his book “The Parish of Zejtun through the ages” writes:
The altarpiece features the Virgin Mary and the Child Jesus giving the scapular to St. Teresa of Avila and to St. John of the Cross. In the lower corner are displayed the Holy Souls of Purgatory.