1) It seems to be currently accepted that the inspiration for Guy Peellaert's cover art is:




2) And there is at least one source that claims that Guy Peellaert himself confirmed this

3) While I am not disputing that the Hitler photo was likely influential, to me its
obvious, from the color scheme and layout, that a famous 19th century painting
of Czar Alexander's coronation also served as an inspiration





4) With regard to Peellaert's women, I would like to add that they are specifically "vestal
virgins", women of purity who had a priest-like function in ancient Rome, especially for
public ceremonies. I haven't found Peellaert's specific source, although
the ladies appear to be circa 1920, when it was not unusual for women to costume
themselves in this manner for, again, public ceremonies...

Footnotes:

Just how virginal the Vestal Virgins were depended on the Emperor

The pic above is from the coronation of Nero

Like with the "Shine a Light" movie, Nero's girls would often score front row seats...








...and after the show, the girls definitely liked to party, even while Rome burned





And now, a different kind of Roman party, the opposite of a coronation, but with a
similar assembly -- the Emperor Vitellius being dragged through the
streets by the angry populace...




And of course it was the Romans who gave the King of Kings the same treatment





As you probably know, "heil hitler" is just the continuation of the timeless
Roman salute "hail caesar" 

Pre-hitler, it was even part of the US pledge of allegiance