Ginger Barfield, Dean and vice president for Academic Affairs at Lutheran Theological Southern Seminary in Columbia, South Carolina, has good insights into this Sunday's Gospel lesson, here.
The painting to the left by Matthias Grünewald is historically inaccurate. Obviously, the execution of John the Baptist (by beheading) occurred before the execution of Jesus by crucifixion. But Grünewald's portrayal as one who pointed to or witnessed to the Lordship of Jesus is accurate. You can click on the illustration to see it enlarged.
By the way, don't confuse John the Baptist with John, the writer of the Gospel of John.
Be sure to read Barfield's short commentary. She does a good job.