In the Monkees newsgroup alt.music.monkees, a poster by the name of Markshark989 pointed out the possibility that Peter Tork's image might have been removed from the sleeve artwork. My curiosity piqued, I had a look. Here's some possible proof.
First, note the dark shape down to the right of Davy's head (he almost seems to be looking in that direction).
Now, move your mouse over the orange part of the sleeve to see an artist's impression of the missing image of Peter. The photo used here is from the same time period as the images of the other Monkees, circa 1968.
Another clue (see enlargement below) is found on bottom right hand corner of the sleeve, showing printed text being cut off at the border of the orange area.