The cover art is credited to Philippe Munch and Jame's Prunier, who have also collaborated on other book covers. Munch is a longstanding illustrator of fantasy works (among other projects) and apparently is a friend of John Howe, who has a long pedigree in Tolkien art.
The figure in the foreground is certainly an orc. He looks similar to Munch's rendering of Uglúk in an interior illustration for the matching edition of The Two Towers (Gallimard, 2008).
He looks like he might be a superior sort of orc, from the fancy helmet, but there are few other clues. Nor are there many candidates for him to be one of the named orc characters. I might guess Gorbag, from Minas Morgul, because Sam notes that his group have better-made armour than the orcs of Cirith Ungol, and the skull clasp is something like Tolkien's description of the Morgul insignia "a moon disfigured with a ghastly face of death". But it could equally be a generic orc.
Munch's illustrations of the Black Riders / Nazgûl do not quite match the figure on the right. He shows the Witch-King on Weathertop as wearing an angular crown, and an earlier cover (Gallimard, 1988) has the Nazgûl in black cloaks and sculptural headwear, with skull faces -
(The flying beast shown here is perhaps not entirely what other readers would imagine.)
These are similar to the Nazgûl seen on the cover of the matching Folio Junior edition of La communaute de l'anneau, who have antler-like bits coming out of the tops of their heads, and glowing eyes. That seems to be a consistent feature of how Munch depicts them.
Nonetheless, the spiked helmet with full face covering is reminiscent of the Witch-King ready for battle, even if the red cloak does not quite fit. If the orc is Gorbag, then this could be the lord of Minas Morgul, with the background perhaps representing Morgul Vale (a steep valley with a stream at the bottom, and ominous ruins on the mountainsides). If his cloak were black then it would be a closer match, though artistically the composition would make him harder to see.
It could still be Sauron himself but I am guessing not, since we don't really think of Sauron as wandering around away from the Dark Tower.
Textually speaking, the Witch-King should have left with his army at the end of the previous book, and is no longer available to brood menacingly over the Ephel Dúath. However, we could also say that Munch's cover of the first book makes a similar error: it shows Gandalf with members of the Fellowship (e.g. that's surely Gimli with the axe), fleeing two mounted Ringwraiths, which does not happen in the book. The Ringwraiths are unhorsed on the way into Rivendell and do not appear again in that form; the Fellowship is only formed at Rivendell, and Gandalf was alone in his prior wraith-encounters. These covers are more about depicting a mood than a specific moment. And I conclude that the artists here were showing the threat of Mordor, with reference to the Witch-King's dominion over Minas Morgul, but without being totally faithful to textual detail.