If you are using Plain Text to compose a message, it is impossible to embed an image.
The problem is with terminology. It is impossible to "embed an image" no matter what you do with an email. Images are
always attachments; whether it appears within the flow of the mail message or not is simply a matter of how the receiver's mail program decides to display it. The HTMLness or non-HTMLness of the message has nothing to do with it -- except that it's possible for an HTML message to refer to an attached image. A plain text message can still have an attached image displayed inline.
The sender's mail program suggests whether to display an attachment inline using Content-Disposition. Many standards-compliant mail programs will heed the suggestion. However, if the receiver can't save the file because it is shown inline, the only problem is that the receiver's mail program is broken.
Apple Mail, unfortunately, is really quite bad when it comes to message composition and allows you almost no control over anything in the outgoing message. So, unfortunately, although it's perfectly possible to work around the flaw in the receiver's mail program by setting Content-Disposition to "attachment", you can't do it with Apple Mail. The problem is that the receiver's email program is badly designed, and Apple's mail program doesn't allow access to what you would need to work around the other program's flaws.