Here’s how it works:

Attaching the Image

When you attach an image to an email, it becomes a separate part of the MIME (Multipurpose Internet Mail Extensions) structure of the email. Each part of a MIME email has its own headers, such as Content-Type, Content-Disposition, and Content-ID. For example, an attached image might have headers like these:

Content-Type: image/jpeg; name="image.jpg"
Content-Disposition: attachment; filename="image.jpg"
Content-ID: <image123>
Content-Transfer-Encoding: base64

Referencing the Image in HTML

In the HTML body of the email, you can reference the attached image using the img tag with a src attribute that uses the cid: scheme followed by the Content-ID of the image:

<img src="cid:image123" alt="Embedded Image">

In this example, image123 is the Content-ID assigned to the image in the MIME part headers. When the email is viewed, the email client will display the attached image in place of the img tag.

Disadvantages

Email Size: Attaching images can increase the overall size of the email, which might affect loading times and bandwidth usage for recipients. Spam Filters: Emails with attachments are sometimes more likely to be flagged by spam filters, especially if the attachments are large.