Michael Whelan is a famous SF artist, best known for his works on science fiction book covers from the 1985 on. His style is sometimes classified as imaginative realism, in parallel to magical realism, as his art is very detailed and realistic... apart from all the dragons and spaceships.
The odds are that you will best recognize Whelan from his covers for the Dragonriders of Pern series (the 80 and 90s republications), which were strongly colored, striking, and immediately drew the eye. At the time they were unusually realistic, and remain some of the most familiar and striking dragons in science fiction. Earlier covers of the Pern series tended towards simple, surreal art, with a strong hint of medieval imagery; the contrast was striking, and it helped make the series popular with a new generation, and to make Whelan one of the most popular artists of the next two generations.
If Dragonriders doesn't strike a bell, he also illustrated the Fuzzies for Ardath Mayhar's Golden Dream series, the cover of Robert Heinlein's Job: A Comedy of Justice, Robert Silverberg's The New Springtime, Stephen King's The Gunslinger and The Dark Tower, and dozens of others.
It is somewhat difficult to give good examples, however, because much of his art was used on later republications of an author's work. For example, if you are familiar with Michael Moorcock's Elric of Melniboné series, the cover you think of first is probably Whelan's, but there are at least half a dozen alternate covers. Likewise C.J. Cherryh's Chanur series (Whelan's covers are the ones where the Hani have natural expressions, rather than uniform angry grimaces) and Poul Anderson and Gordon R. Dickson's Hoka series.
While it is hard to characterize Whelan's art, especially in words, he is known for works that are not just realistic, but that also manage to convey a sense of awe and wonder. He likes strong contrasts, and often uses bright colors and strong lighting, which gives a unreal and lighthearted sense to many of his works without requiring any compromise on the detail or setting. Perhaps most important to his success, his art works very well as stand-alone pieces, and are not designed to be a background to a title, author and blurb combo. As a consequence, his work makes book covers not just grabbing, but continuously interesting under close inspection; in fact, if you like one of his covers, I would recommend looking for a clean copy of the work, as it will surely look better without words all over it.