JOEL - According to the Gospels all twelve Apostles were there at the beginning of the evening(Mat 26:20, Luke 22:14). In Luke, after administering the bread and wine, Jesus said, "the hand of him that betrayeth me is with me on the table"(Luk. 22:17-21). Then, according to John, Judas left sometime during the evening when Jesus identified him as the traitor(John 13:21-30), after which Jesus continued on with His instruction(John 13-15).
So according to Luke it appears that Judas was there for the bread and wine sacrament. But many do not agree with this interpretation, unable to believe that Jesus would allow someone to partake of the sacrament unworthily. And many Bible scholars seem to agree that no one can be sure of the timeline or order of the events as they really happened.
According to LDS Apostle James E. Talmage, who wrote "Jesus the Christ":

"Did Judas Iscariot Partake of the Sacrament of the Lord's Supper?-This question cannot be definitely answered from the brief accounts we have of the proceedings at the last supper. At best, only inference, not conclusion, is possible.
To many it has appeared plausible, that because of his utter baseness Judas would not be permitted to participate with the other apostles in the holy ordinance of the Sacrament; others infer that he was allowed to partake, as a possible means of moving him to abandon his evil purpose even at that late hour, or of filling his cup of iniquity to overflowing. The writer's personal opinion is based on the last conception." (James E. Talmage, Jesus the Christ, 1983, p. 573)

Most artists paint the scene with Judas still there and some choose to paint the scene at a point in time after he left. So it is true both paintings could be correct depending on how the scriptures are interpreted. Leaving Judas out of the painting is simply a matter of personal choice of some artists who feel that Judas should not have been involved in the sacrament.