The first time I heard 'Imagine' was around 1973 when I was 11. And it was Diana Ross' version from the 'Touch Me In The Morning' album.
As far as I remember the original was known as the title track of the album but was released as a single -with 'Working Class Hero' on the B side.
https://www.youtube.com/watch?v=9gMEzHcSH4M