Embed a picture in the body of an email message

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If all you need is for the image to be visible to the recipient, most mail clients will display ordinary image file attachments at the end of a message.

Otherwise, you're talking about sending a html (or "rich text" in Apple's jargon) message.

Click the "Rich formatting" button in Gmail to compose an html-formatted message.

Be aware that some people's mail clients and spam filters will block your message as spam.