Imagine John Lennon on MTV. Purists might think video would kill or compromise the late radio star's appeal, but this visual greatest-hits document gives the lie to that popular misconception. Assembled from a wealth of archival footage, live performances, and home movies, Lennon Legend feels surprisingly cohesive, marrying affecting images to indelible classics like ''Imagine,'' ''Mother,'' and ''Working Class Hero.'' As you might expect, there's lots of footage of John and Yoko walking around engaging in love-me-do-type turtledovin', but somehow it's touching rather than schmaltzy. And check out the wonderful scenes of a loosey-goosey Lennon clowning around in New York's Central Park band shell in ''Mind Games.'' Nostalgia at its niftiest.