I have found the solution to the JPG problem of - Unable to display JPG files with 'Windows Picture & Fax Viewer' on a PC, sent from a MAC .
The problem is due to MS Entourage and it setting the encoding option to 'appledouble' instead of just 'mime'.

a Mac user should go to---> Mail & News Preferences---> Compose and change the Encode for: (Under 'Attachments') to 'Windows (MIME/Base64)'.