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.
Hm, I haven't encountered this problem before, but it might be due to the formatting Mail.app does to the messages. Try making the message plain text by clicking "Format" in the menu bar and selection "Make Plain Text".
Probably not the best solution, but you could zip the images before sending them (right-click and choose
Compress ... on OS X). Archiving and unarchiving zip files is supported natively in Windows since XP (I think), so even non computer-savvy people should be able to open your attachments.
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).