The Obtuse Arguments of Lady Boyle is one of Anton Sokolov's paintings and a collectible in both Dishonored and Dishonored 2.
In Dishonored
The painting is found inside Sokolov's House, on the lowest level.
When you capture him, Sokolov says he only painted Lady Boyle from the rear, and as such does not know which of the three Boyle sisters (Esma, Lydia and Waverly) she is.
In Dishonored 2
The painting is found in Dr. Galvani's Apartment, in a room guarded by a City Watch guard. Behind it is the combination to the safe in the same room, a random 3-digit combination. It is worth 200 
.