QUESTION:
Why is it that in most NASA and Pathfinder pictures there is normally a
corner of the picture missing?

ANSWER from David Mittman on July 25, 1997:
While I haven't seen a specific picture that had a corner missing, there are
a lot of pictures that have missing pieces, usually vertical black bars.
These black areas show up when we lose a part of the picture data as it
travels between the spacecraft and Earth. Each picture gets split up into
between 3 and 155 different packages of information. Any of those packages
can get garbled or lost due to a communications problem. At the end of each
day we gather together a list of all the packages of data. The next day we
tell the spacecraft to retransmit those packets which got lost. Most of the
images with missing pieces will be reprocessed later when the missing data
has been received.