Elliott (1982) states that Carl disagreed with a suggestion to incorporate an image of Bruce into the selected gatefold photo as he didn't regard Bruce's membership equal to that of the other members of the group (p. 17).
References
Elliott, B. (1982). Sifting sands: Ten years of harmony. Add Some Music, 5(1), 1-50
I believe there was a compromise offered, that of including a smaller, separate image of Bruce, but he declined.
True. Bruce decided that if his image wasn't welcome in the main photo then he didn't want to be represented photographically on the album at all.