Stephen’s story is in Acts 7. Paintings of his martyrdom are not common by any means, but this and the later one at Catfield in Norfolk (link below) have survived. Even in the painting’s faded state, enough can be seen of his robes to confirm that he is correctly dressed as a deacon of the church as he kneels facing right in prayer. The legs of a torturer are still just visible to the right, but the other torturer on the left, about to hurl a stone, is fairly clear in his entirety. He has a great bag of stones, all of more or less uniform size and shape as was apparently prescribed, slung around his waist¹. He also has interesting hair - a shock of it standing out around his head, especially on the left. ‘Bad’ characters in medieval narrative painting often have wild hair - Cain’s, in a painting of Cain and Abel at nearby Kelmscott and roughly contemporary with this, is similar.
Some other paintings at Black Bourton are in the table below. There are more, including the Coronation of the Virgin, the Adoration of the Magi and the Baptism of Christ all of the same 13th century date.

¹Or, alternatively, holds the stones in the tucked-up lap of his outer garment.

St. Andrew Martyred, Stoke Dry, Rutland	St. Anne teaching the Virgin to read-Corby Glen, Lincolnshire	St. Antony and the Pig, Barton, Cambridgeshire	St. Barbara : Hessett, Suffolk	St. Bartholomew : Selling, Kent	St.Catherine of Alexandria, life of : Castor, Cambs	St.Catherine of Alexandria : Hardley Street, Norfolk
St.Catherine of Alexandria : Old Weston, Northants	St.Catherine of Alexandria : Pickering, N. Yorks	St.Catherine of Alexandria, life of : Sporle, Norfolk	St. Clement : South Leigh, Oxon.	St. Dunstan holding the Devil by the nose : Barton, Cambs	St.Edmund : Boxford, Suffolk	St. Edmund : Lakenheath, Suffolk
St. Edmund, Martyrdom of : Bishopsbourne, Kent	St.Edmund, Martyrdom of : Fritton, Norfolk	St. Edmund (or St. Walstan) : Gisleham, Norfolk	St. Edmund, Martyrdom of : Pickering, N.Yorks	St. Edmund, Martyrdom of : Stoke Dry, Rutland	St. Edmund, Martyrdom of : Troston, Suffolk	St. Edmund, Martyrdom of : Weare Giffard, Devon
St.Eloi, Broughton, Bucks	St. Eloi and the possessed horse, Slapton, Northants	St. Eloi, as bishop & blacksmith, Wensley, N.Yorks	St.Erasmus, Martyrdom of : Chippenham, Cambs	St. Etheldreda : Willingham, Cambs	St.Francis Preaching to the Birds : Wissington, Suffolk	St. Francis Receiving the Stigmata, Slapton, Northants
St. George & Dragon : Banningham, Norfolk	St. George & Dragon : Broughton, Bucks	St. George & Dragon : Earl Stonham, Suffolk	St. George & Dragon : Fritton, Norfolk	St. George & Dragon : Hornton, Oxon	St. George dedicating himself to the Virgin : Astbury, Cheshire	St. George, with the princess : Little Kimble, Bucks
St.Helena, Broughton, Bucks	St. Hubert, Miracle of : Idsworth, Hants	St.James the Great : Hales, Norfolk	Life of St. James, Stoke Orchard, Gloucestershire	St. James the Great, meeting pilgrims : Wisborough Green, Sussex	St. John the Baptist, Martyrdom of : Chalfont St. Giles, Bucks	St. John the Baptist, Martyrdom of : Heydon, Norfolk
St. John the Baptist, Martyrdom of : Idsworth, Hampshire	St. John the Baptist, Martyrdom of : Old Weston, Northants	Life of St. John the Baptist : Cerne Abbas, Dorset NEW	St. John the Evangelist, Selling, Kent	St. John the Evangelist, Weston Longville, Norfolk	St. Margaret of Antioch : Old Weston, Northants	St. Margaret and the dragon : South Newington, Oxfordshire NEW
St. Margaret of Antioch Martyred, Stoke Dry, Rutland	St. Margaret of Antioch, Life of : Charlwood, Surrey	Life of St. Margaret, Wendens Ambo, Essex	St. Martin dividing his cloak, Chalgrave, Beds	St. Martin dividing his cloak, Wareham, Dorset	St.Nicholas of Myra, life of : Little Horwood, Bucks	St. Nicholas of Myra, two miracles of : Wissington, Suffolk
St.Paul : Black Bourton, Oxon	St. Paul : Beckley, Oxon	St. Peter : Beckley, Oxon	St.Peter : Black Bourton, Oxon	St. Peter, Martyrdom of : Chacombe, Northants	St. Roch : Pinvin, Worcs	St. Sexburga? : Willingham, Cambs
St. Stephen, Stoning of: Black Bourton, Oxon	St. Stephen, Stoning of: Catfield, Norfolk	St.Swithun (?) enthroned : Old Weston, Northants	Scenes from the life of St. Swithun : Corhampton, Hampshire NEW	St. Thomas Becket, blessing, Hauxton, Cambs.	St. Thomas Becket, Murder of, Marston Magna, Somerset	St.Thomas Becket, Murder of : South Burlingham, Norfolk
St. Thomas Becket, Murder of : South Newington, Oxfordshire	St. Walstan of Bawburgh? or St. Edmund : Gisleham, Norfolk	St. Zita : Horley, Oxon	St. Zita : Shorthampton, Oxon	Unidentified female saint : Heydon