The artwork on the cover of my copy of The Silmarillion is a drawing by Tolkien himself. It shows a huge mountain silhouetted by a blue moon, witha Viking-style ship in the lower foreground.
What is this a picture of?
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It appears to be a stylized version of a watercolour by J.R.R. Tolkien titled "Taniquetil":
"Taniquetil" was painted in 1927, and depicts several elements (not all of which are included on the stylized cover):
Most prominently, the mountain Taniquetil, highest of the mountains of the World and home of Manwë,Lord of the Valar:
[A]bove all the mountains of the Pelóri was that height upon whose summit Manwë set his throne. Taniquetil the Elves name that holy mountain, and Oiolossë Everlasting Whiteness, and Elerrína Crowned with Stars, and many names beside; but the Sindar spoke of it in their later tongue as Amon Uilos. From their halls upon Taniquetil Manwë and Varda could look out across the Earth even into the furthest East.
The SilmarillionIII Quenta Silmarillion Chapter 1: "Of the Beginning of Days"
The glowing halo of light at the peak of the mountain is presumably meant to represent Manwë's throne, or alternately the power of God; I find it visually similar to Gustave Doré's illustrations for Dante's Paradiso, especially "Celestial Rose":
The foreground depicts one of the Swan-ships of the Teleri (who would have still been the Solosimpi at this stage of the mythology):
For [Alqualondë] was their city, and the haven of their ships; and those were made in the likeness of swans, with beaks of gold and eyes of gold and jet.
The Silmarillion III Quenta Silmarillion Chapter 5: "Of Eldamar and the Princes of the Eldalië"
Although I'm not sure of confirmation, the layers of sky around Taniquetil are reminiscent of the Three Airs, particularly the 1930s-ish version described in the Ambarkanta:
Within these walls the Earth is globed: above, below, and upon all sides is Vaiya, the Enfolding Ocean. But this is more like to sea below the Earth and more like to air above the Earth. In Vaiya below the Earth dwells Ulmo. Above the Earth lies the Air, which is called Vista, and sustains birds and clouds. Therefore it is called above Fanyamar, or Cloudhome; and below Aiwenore or Bird-land.
Ilmen is that air that is clear and pure being pervaded by light though it gives no light. Ilmen lies above Vista, and is not great in depth, but is deepest in the West and East, and least in the North and South. In Valinor the air is Ilmen, but Vista flows in at times especially in Elvenhome, part of which is at the eastern feet of the Mountains; and if Valinor is darkened and this air is not cleansed by the light of the Blessed Realm, it takes the form of shadows and grey mists.
History of Middle-earth IV The Shaping of Middle-earth Chapter 5: "The Ambarkanta"