Morrissey was the one who chose the images for all The Smiths' record sleeves when The Smiths were still clients of Rough Trade Records. Morrissey's personal assistant at Rough Trade Records was Ms. Jo Slee. In August of 1994, she published a book which expounded on all of the cover images that Morrissey had chosen. Her elaborately illustrated book was titled "PEEPHOLISM: Into the Art of Morrissey".

PEEPHOLISM: Into the Art of Morrissey.
Published in August of 1994.
Published in August of 1994, PEEPHOLISM had this to say about the image of Diana Dors before it wound up on the cover of The Smiths "SINGLES" compilation released by Warner Brothers:
"A
year or two before The Smiths catalogue was sold to Warner Bros., a still of
Diana Dors from the film YIELD TO THE NIGHT (1956) was considered for a
'greatest hits' compilation then being incubated by Rough Trade".
- PEEPHOLISM. Page 75. Published
in August of 1994.

- PEEPHOLISM. Page 75. Published in August of 1994.
Thus, it became public knowledge that Morrissey was considering using that image of Diana Dors for what would have been the next Smiths reissue from Rough Trade Records. That reissue never materialized because the rights to The Smiths catalogue were sold to Warner Brothers Records. Taking their cue from Morrissey, Warner Brothers used the Diana Dors image one year later as the cover of the final Smiths reissue before Princess Diana's death:

The Smiths "SINGLES" compilation.
Released in May of 1995.
Morrissey had this to say in January of 2006:
"I designed all the Smiths
original releases - but not the post-split re-issues [although the Diana Dors
"Singles" sleeve was an image I had been hoarding.]"
- Morrissey, January 4, 2006
Source: http://true-to-you.net/questions_with_answers_from_morrissey
The bottom line is this:
If you ask, "Where did Warner Brothers get the idea to use that photo of Diana Dors?", the answer would be, "Warner Brothers took the idea from Morrissey".
____________________________________________________________________________________________________