The altarpiece is a white marble relief sculpture by Francesco Brunetti, 1685. It shows a group of angels holding an icon in a frame, which is being venerated by SS Joseph (on the left) and Anthony of Egypt (kneeling). The icon itself of the Madonna and Child is a cut-down fragment of a larger work, which looks 15th century. Above is the Dove of the Holy Spirit in a gilded glory.
(Another photo of the icon here)