This works. Try it.
In my case, I wanted my images encoded as base64 and inline... not as an attachment.
Create a new email in Apple Mail. Address it to yourself and give it a subject. Drag an image into the body of the message. Don't click the attach button just use the drag and drop method. Send it.
Now do the same thing with another email except this time. Type something in the body, then drag an image into the body.
Open both emails you receive from yourself and view the source, side by side.
If an image is in the first thing in the body of an email it will be encoded inline as base64 other wise, it is plain text.
Using the "Attach" button makes sure that the image is always encoded properly in the body no matter where it is. When I drag an image into the email (or in my case I was having Applescript do it for me) if there was anything before the image in the body, the image would not be encoded properly for MMS/SMS.
Why is this important? If you want to send and SMS/MMS via email, the image must appear first or it is stripped out. This may also be important for other email services as to how they treat embeded content.