Hi,
this 'bug' is due to Thunderbird itself, I guess, but it is related to Contact Photos, so you may want to investigate it.
Apparently, in Fedora 12 (KDE or Gnome, two different computers checked), the default photo in the Address Book is missing. The 'Generic Photo' field has still 'Default' (and no other choices are present), but the picture is missing.
Therefore, when you receive a mail from a sender without a photo, Contact Photo does not even create a box on the left, but only a small line.
Stefano