Thank you !
I was trying to find the mirror on the left corner, but I can't.
I attended the conference "An evening with Four5ONE" and they said:
"in the lower left corner of the inner photo of The Joshua Tree -- the photo of the four band members with the tree in the background -- you can see a mirror that the band had been using to check their appearance during the shoot."
You can't really see it in this picture either, but check out a vinyl or the cd booklet for the JT. It's right in the lower left-hand corner of the picture.