This specific question has been debated heatedly since book covers took the limelight instead of author names. Even though this issue affects almost all genres, here I am only considering romance books that use torsos(and not the ones with houses, wedding rings e.t.c as models on the cover usually represent a certain level of steaminess) because they usually depend completely on single images and typography.
I was scrolling through Facebook some days ago when I saw a post by Sylvia Frost, author and cover designer, that said that with the rise of Indie publishing, the old stereotypes of not showing the face is breaking. From my own experiences as a cover designer, this seems to be true. The reason faces were not shown in the first place was because they would affect the reader’s imagination of what the characters look like. But the formula that was applied at a time when the market wasn’t saturated with millions of books cannot be applied now. With e-books being the main source of income for most authors, they only have a single second to capture the attention of the reader. Psychologically speaking, we are naturally drawn to faces, especially human ones. And the feature that draws us in the most are eyes. So, in the current market, in order to quickly grab your reader’s attention it’s better to feature the face—unless you are a well known author, in that case it will mostly be your name selling the book.
To new authors, I suggest you give the faceless cover a rest and try one with a model that you think best represents your character.