What can be lacking in the sufferings of Christ? I remember being taught that whatever sins Christ did not suffer (atone) for cannot be forgiven.
A beautiful modern theological interpretation of this question is John Paul II's Encyclical Salvifici Doloris. In it the pope affirms the perfection of Christ's suffering on Calvary, but calls them accomplished, not completed.
While we must avoid any Pelagian tendencies to think our sufferings earn our salvation, we can nevertheless actively join our own sufferings to Christ's and become not simply spectators at Calvary, but true participants. Linear time does not matter here: Just as Christ suffered to atone for sins we have not yet committed, so can we join our sufferings to his even though His crucifixion on Calvary is history. The sufferings of Christ on Calvary are both outside human history and yet intimately enfolded within it.
The good Sisters used to tell their pupils, "Offer it up." The pope even mentions this practice in his encyclical and embraces both the spirit and truth of the admonition, if not its simplicity. I return to this encyclical over and over because its truths are so important in accepting the blessings and meeting the challenges of Christian discipleship.
The painting is the part of the Isenheim Altarpiece (c. 1512) by Matthias Grunewald. It was remarkable then and now for the intense depictions of the Christ's sufferings. It was painted for a monastic chapel whose monks cared for the sick and suffering.