Memento mori translates from Latin as "remember you must die". A portrait with a skull would be the standard memento mori painting, although other frequently seen symbols include hourglasses or clocks, extinguished or guttering candles, fruit, and flowers.

“Accepts death in a cheerful spirit, as nothing but the dissolution of the elements from which each living thing is composed. If it doesn’t hurt the individual elements to change continually into one another, why are people afraid of all of them changing and separating? It’s a natural thing. And nothing natural is evil.” 

-Meditations, Marcus Aurelius