This is a test of embedding an image in HTML and relate the img tag in the HTML document with the image by mailsend while sending mail. We will create an img tag in the HTML document with: 
            <img src="cid:xx_deadbeefcafe" alt="inline imge">

inline imge

The real HTML file is test/embed_image1.html, available with the source distribution.

Notice: only content id is specified with cid, no path of image is specified.

While sending mail with mailsend, we will specify the content id xx_deadbeefcafe with flag -content-id for the image.

Example:

Note: Requires mailsend v1.17b15+. The following command was used to send this mail: 

    mailsend -v -sub "Testing embedding image in HTML" 
    -from example@gmail.com -to example@gmail.com 
    -smtp smtp.gmail.com -port 587 -starttls -auth 
    -user mailsend.test@gmail.com 
    -cs ISO-8859-1 
    -content-type "multipart/related" 
    -mime-type text/html 
    -disposition inline 
    -enc-type "none" 
    -attach "test/embedded_image1.html" 
    -mime-type image/png 
    -enc-type "base64" 
    -disposition inline 
    -content-id "xx_deadbeefcafe" 
    -cs "none" 
    -attach "test/blue.png"

Any mail reader that understands multipart-related Content-type will display the html and the image together. Without the content id, the HTML and the image will show up as 2 separate attachments.

Tested with gmail, outlook and yahoo mail which can display HTML file with embedded image.