If just one earring is visible, then the title might very well be "Girl with a pearl earring." The point is to tell you what you will see in the picture and what your eye should be drawn to. If you are looking at the picture, my guess is that your gaze will be drawn first to the earring that is visible, then move to the rest of the picture. The composition of the picture is meant to flow visually from the earring. The fact that the girl is probably wearing two earrings, one of which is not visible, is unimportant as far as the effect the picture was meant to have, visually and emotionally.

Girl with a Pearl Earring is quite a famous Dutch painting. The title is a singular "earring" because you see the girl with her head turned and you can only see one earring in the view of the left side of her face. The pearl earring is the focus of the painting and the title accurately reflects the singular earring. You can say that "I have lost an earring", it does not have to be plural. An work of fiction by the same name by the authoress Tracy Chevalier is well worth reading as she has footnotes that explain some of the symbols seen in the painting. The book is a very good light read and I highly commend it.