That may be an effect of any anti-virus that checks incoming and outgoing emails.
It may be that anti-virus scanners rip images and such out of emails to scan them, then add them back as attachments.
I am using Outlook 2003. My email editor is Word 2003 and I use the HTML format option for emails. When I insert a picture in the body of an email message, the sent email shows the picture in the message but also has an "image001.jpg" file attached to the sent email. How do I stop Outlook 2003 from adding this image attachment to the sent email?