Complex sentence: The Streatham portrait may have been part of a collection of paintings of Protestant martyrs, and damage to the painting's mouth and eyes suggests that at one time it was attacked by an unknown individual, probably not long after completion.
Split into simpler sentences: The Streatham portrait may have been part of a collection of Protestant martyr paintings. Damage to the painting's mouth and eyes suggests that at one time it was vandalised, probably not long after completion.