Looks cranky. Some Christian motifs may have been ripoffs from other religions, but it's the first time I'm hearing this about Jesus and crucifixion.

There are non-Christian sources mentioning the crucifixion of Jesus within few decades afterwards.

If they think that crucifixion was a common motif in religions of the time they would better provide some examples. Their /cr.htm just repeatedly states it as fact.

No, this "Orpheus crucifixion" stone doesn't count because it was probably created by Roman Christians who were known to combine Jesus with Orpheus in their art due to the obvious parallels. And the original Greek myth of Orpheus says nothing about crucifixion.