​
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.