Let me guess Outlook 2003? JPEG attachements sent from an Apple user?
I had the same problem recently and in our case it was a problem with
Mac users sending pictures from Entourage. If you have any influence
on the Mac person have them go to Entourage - > Preferences and select
Mail and News composition. The encoding method needs to set to
Windows (Base64\MIME) instead of AppleDouble. The misleading part is
that AppleDouble is listed as working on "all computers." MIME
encoding will not prevent the Mac users from being able to open
pictures sent to them from other Mac users.