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

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+: 

    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 mailsendreal@gmail.com 
    -cs ISO-8859-1 
    -content-type "multipart/related" 
    -mime-type text/html 
    -disposition inline 
    -enc-type "none" 
    -attach "/home/muquit/svndev/mailsend/test/embedded_image.html" 
    -mime-type image/png 
    -enc-type "base64" 
    -disposition inline 
    -content-id "xx_deadbeefcafe" 
    -cs "none" 
    -attach "/home/muquit/svndev/mailsend/test/test.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.