Oh, the good old days of singles. MK, has not issued one for privateering and/or the Get lucky ones were only promos.
But in a sense, all singles from the 70ies on were in reality a promotional item (and cheap, barely covering the expenses) in order to convince you to buy the actual product which was the album.
And it is true that MK does NOT appear in any of the album (solo or DS) front covers, except live at the BBC.
But the singles were a taste for the newcomers and an extra gift for the fans. So the artwork control was not as strict as the albums'. You get to have several different covers, with all sorts of styles and aesthetics. And of course photos to thrill the fans, to make more intimate and to familiarize the newcomers.