In the absence of a processional crucifix or any other nearby crucifix, would a depiction on the wall of the descent from the cross (where Christ is already on the ground) satisfy the requirement?
In my opinion, No.
From the General Instruction of the Roman Missal Chapter V
308. There is also to be a cross, with the figure of Christ crucified upon it, either on the altar or near it, where it is clearly visible to the assembled congregation. It is appropriate that such a cross, which calls to mind for the faithful the saving Passion of the Lord, remain near the altar even outside of liturgical celebrations.
It might make a very beautlful altarpiece (remember them?) but does not fulfill the requirements for a crucifix.
My parish has a crucifix of the risen Christ behind the altar on the wall. Is this valid in the absence of the processional cross or must a crucifix depicting Christ crucified be used?
No. It needs to be Christ crucified. Not only does this violate the GIRM, it also goes against St. Paul’s words wherein he said that he preaches Christ crucified, as I see it.
Does it depict Christ crucified, “nailed to the cross” ?
So this invalidates the Mass at my parish if the processianal cross is not used
It doesn’t “invalidate” the Mass, it is still really and truely a Catholic Mass.
What is means is that the Mass is lacking in a certain way as mandated by Rome.
It lacks the symbolic unity of image between the Sacrifice of the Cross and the Sacrifice of the Mass.
They are really one and the same Sacrifice and thus the Church desires the symbolic reminder of having the image of Calvary present.
Does it alck mean that the Sacrifice of the Mass doesn’ happen? No.
What it means is that it could be better, and, as Catholics, we are to seek that which is better for us.
It does not invalidate the Mass. However it is your pastors responsibility to insure that a Crucifix is visible on or near the altar during the celebration of Mass.