P.S This answer does not deserve any upvotes! It is very deficient regarding non-English publications. Namely, in my haste, I only searched under the title "Dune" and neglected to search under the various translated titles, listed on the ISFDB title page for the novel. If you already upvoted, you can retract it now that I've edited my answer.
All the information in this answer is from the Internet Speculative Fiction Database.
On this page the ISFDB displays the covers of all editions of Dune that it's aware of. This includes many recycled paintings, and also some plain covers with no picture.
An advanced title search with search criteria "Title is exactly dune" and "Title Type is exactly COVERART" retrieved this list of 23 works, which still contains a couple of duplicates (variant titles or forms of the artist's name). After eliminating duplicates I came up with a list of 20 or 21 different works of art by 18 artists, listed below. Images of #10 and #15 were not available; possibly #15 duplicates #12.
Note that this is mainly a list of cover art on English-language editions. However, since I failed to specify "Title Language is exactly English" in my search, it accidentally includes 3 non-English editions (#7, #9, #14) which bear the original title Dune. Many of the titles of translated editions are listed on this ISFDB page.
- Henrik Sahlström, date unknown link
- John Schoenherr, 1965-01-00 link
- John Schoenherr, 1965-03-00 link
- John Schoenherr, 1967-00-00 link
- Bruce Pennington, 1968-07-00 link
- Don Punchatz, 1974-08-00 link
- El Cubri, 1975-06-00 link
- Vincent Di Fate, 1975-09-00 link
- Angus McKie, 1982-10-05 link
- Frederic Marvin, 1983-11-00 link
- Bryn Barnard, 1984-04-16 link
- Gerry Grace, 1988-00-00 link
- George Steinmetz, 1990-09-00 link
- Dan Alexandru Ionescu, 1992-00-00 link
- Gerry Grace, 1999-00-00 link
- Grey Thornberry, 2001-00-00 link
- Margo Goody, 2007-08-00 link
- Sam Weber, 2015-00-00 link
- Francis O'Connell, 2015-07-16 link
- Alex Trochut, 2016-00-00 link
- Matt Griffin, 2019-10-01 link