It began for Nadia Matievskaia, an artist from Belarus, when an art class assignment required her to paint with her breasts.
(That seems like a big deal, but I can't find any details about the incident.)
She enjoyed the experience and began using her breasts in addition to traditional brushes. This video shows how she does it (warning: nudity).
Matievskaia posts the results on her Instagram page (warning: artistic nudity). Considering the unwieldiness of her artistic implements, she's able to achieve a surprising level of detail and precision. I could certainly do no better.
This painting is particularly noteworthy. It shows Alexander Lukashenko, the dictator of Belarus.
-via Oddity Central