I’ve had this problem for a while, that Mac OS X Mail (the native mail client in OS X) sends emails that other email clients don’t recognize as attachments, but as embedded pictures. This has been really frustrating, because if I’m sending an image to a client that needs to be saved on the client’s computer, he or she is unable to do so because the image is not a “real attachment”.
So, here are the facts: Wether an image is embedded in the email or not, it’s still an attachment. Thus, OS X Mail does nothing wrong when it embeds the image, other clients simply doesn’t want to understand that it’s still an attachment. The clients (read: “Outlook”) offer no way to save the image other than to right click it inside the email, but that only generates a .bmp that’s scaled down to the email size in pixel width.
Please note that it doesn’t matter if you click the Attachment button in the main taskbar or simply drag the images to the email body.
The reason my images were getting embedded was because I was using some signatures with colors in them. As soon as you set the font, a color or similar, Mail turns the email into a “Rich Text” email, which basically means it’s a HTML email. So when you attach images to such an email, Mail generates an tag for that image, thus embedding it into the email itself. You can avoid this easily by just using Plain Text emails (see screenshot).
You can also try to simply don’t use any formatting in the email, including your signature.
Hope this helps.