This is Eärendil, setting sail in his swan-prowed Vingelot, out from the dark cliffs with the Silmaril on his brow. This picture was based on a poem in the Book of Lost Tales Part II, in my edition on page 267: Eärendil arose where the shadow flows/At Ocean’s silent brim…
I like the older version of the last verse, though: Tracking the Sun in his galleon/And voyaging the skies/Till his splendour was shorn by the birth of Morn/And he died with the Dawn in his eyes.
It’s much more hopeful and glorious than the newer version, which has him withering in the darkness of the abyss.