The Orcs of the Misty Mountains usually used scimitars. In The Hobbit, the bodyguard of Bolg wielded steel scimitars in the Battle of Five Armies. In The Lord of the Rings, the Fellowship found orc-scimitars with blackened blades in the Chamber of Mazarbul. Later, a high orc-chieftain clothed in black mail with a spear and scimitar rushed into the Chamber. Later at the flight of the Fellowship, Frodo saw that nearly hundreds of orcs brandished scimitars and spears in the firelight of the First Hall, near the Bridge of Khazad-dum.
The goblin-soldiers of Isengard used short, broad-bladed swords instead of curved scimitars. Aragorn distinguished four of them from the slain, likely knowing that Orcs from the North normally wielded scimitars.
The Haradrim were those commonly described as using scimitars, with their cavalry once bringing scimitars onto the Pelennor Fields. In the books, their scimitars were "like a glitter of stars" when drawn.
Tolkien uses the term "scimitar" to describe any curved blade existing in Middle-earth. However, there are other kinds of curved blades, including forwards-curving ones like the Kopis and the Falcutta. Some backwards-curving blades do not count as scimitars in swordsmanship, but in the case of Middle-earth as Tolkien describes it, any curved blade is a scimitar. The scimitar in the picture is actually a falchion; a scimitar is a backwards-curving blade on a J-shaped handle with a cross-guard that curved up on the sharp side and down on the dull side. The blade curve was asymmetrical to the handle curve.