I presume you are using the Mail application; if not, this may not apply.
I think you may be misinterpreting what you are seeing. Mail displays attachments as part of the message if it can understand them, as with PDFs or common image formats. It may look like it is embedded, but it is attached. This can be confusing if you attach an image to an email as Mail will display it wherever the insertion point (= cursor) was in the message at the time it was attached, however, it is still an attachment, and I suspect that even recipients using Mail will see it appended, but I have not verified this.
Actually, short of including some html with a link in your email, I don't know how you would embed an image.