Jean Fouquet
The Martyrdom of St. Andrew

Illustration from the Heures d'Étienne Chevalier
Musée Condé, Chantilly, France

The great multitude standing behind the soldiers may represent the "twenty thousand men" who came to hear St. Andrew preach for two days from the cross.

This is the earliest example I have found of St. Andrew on the X-shaped "cross saltire" that came to be closely associated with him.

Read more about St. Andrew.

Source: Wikimedia Commons.