Term used for the final work of an artist before their death

A swan song is, according to the OED

the last work of a poet or musician, composed shortly before his death; hence, any final performance, action, or effort.

This phrase, although rather metaphorical, captures your desired meaning. It is more often applied to musicians than other artists; however, it can be used in a wider sense.

A slightly informal option is last hurrah, though Merriam-Webster doesn't give it this tag.

a final, often valedictory effort, production, or appearance

From Merriam-Webster

A final act, performance, or effort.

From Oxford Dictionaries.

The adjective valedictory literally means 'a farewell', so this fits nicely. Also, the term can apply to pretty much anyone, so there's no specificity.

Finale means "final composition". It is among the many ways to refer to a painter's last work. Parting expression, final piece, consummate representation, and ultimate rendering all capture the idea of the final painting.

An example:

This painting was his ultimate rendering.
It was his parting expression.