displaying graphics in mht files

After creating Excel workbooks with charts, we save the files as .html files. We opent he files in Explorer 5.1 and save them as web archive (*.mht) files. We then email (using Outlook Express 5.0 or Outlook 2000) the mht file to users. These users are on NT 4.0 ws using Outlook Express 5.0 and IE 5.5. Some users can view the mht files and see the graphics, other users only see the “missing graphic” icon. Anybody have any ideas as to why this may be happening?