where are the pictures of the skype contacts saved (see if someone is invisible)
On Skype 6.3.xx the avatars are stored in

~/Library/Caches/com.skype.skype/(your skype name)/SKAvatarCache


in TIFF format. You can view them by in Preview by opening Terminal and running 

open ~/Library/Caches/com.skype.skype/(your skype name)/SKAvatarCache/(name of the file)