I agree with FuzzyPlum in that I've never thought the band did this intentionally. There are other threads that suggest the photographer might have edited this in, but I've never clearly seen the woman without a prompt. I'm always surprised when the cover makes these hidden meaning lsts.
Here's another old (2003) thread for anyone interested. It had photos and images that are no longer included.
http://ledge.fleetwoodmac.net/showthread.php?t=12264