That is not your issue, it's on the other system. By default, when pictures
and fax viewer is opened, it will allow a user to see any pictures they have
stored in the folder in which they have saved a given file. My guess is,
they are saving the attached file to their My Picture's folder and opening
the attachment there.
It's possible, they are simply opening the appended attachment and the
picture and fax viewer is simply referencing anything else that happens to
be in their My Pictures folder that is of course unless you happen to be
sending all the pictures in your My Pictures folder. Have you checked with
others to see if the images are images stored on their system in their My
Pictures folder or if they are in fact all images from your system?