I already have Windows Friendly attachments enabled, but I'm still having a problem. Whenever I send a image attachment to windows (Outlook) users, it comes as an inline image, not as an attachment. They can't save the image like a regular attachment.
I remember somewhere around Panther (when I still used Mail.app), that Outlook users (on Windows) would have problems with my attachments. I was trying to find the solution I had to apply and I think this link has most (if not all) the information. It essence it was an Outlook’s bug, but there was a fix. It’s explained there.
Regarding the “inline” thing, it’s because Outlook is reading the email as a RichText with inline UUencoded info. As already suggested, try zipping the attachment or send plain text messages to those recipients (there’s a shortcut to easily convert a message to PlainText, see Kyle’s reply).