There is, arguably, only one truly famous depiction of the Last Supper: Leonardo Da Vinci’s. There are hundreds of types of replicas of it and if there is an image of the Last Supper attached to an altar it is almost always this one (I personally have never seen anything). But should it be?
The altar or communion table at a church has an association with the Last Supper as the place where Jesus said “Do this in memory of me.” That’s the connection between the two events. But, of course, the Last Supper was a longer meal (particularly as represented in the Gospel According to St. John). It would seem reasonable to pick an image reflecting this moment.
But this is not what Da Vinci’s last supper depicts. Rather, his image shows the disciples responding to Jesus’ pronouncement that “One of you shall betray me.” They are talking among themselves and Peter is cleaning close to John to have him ask Jesus for more detail.
Not only is this the ‘wrong’ part of the meal, the attitude of the disciples is not one that should be emulated at the liturgy. A close look at the painting makes it apparent that they are all leaning away from Jesus. This is not part of the Last Supper where they feel close, but where they are perhaps most alienated from him. It is the low point of the meal, the moment where everything seems free to go wrong.
Da Vinci’s last supper is an amazing painting and deserves our attention. But that attention should bring us to realize what is being depicted and to thus treat it appropriately. Thus it is not really a good image to attach to an altar or use in relation to communion.