I recently had an issue in a client's development which required images to be embedded in html email code (<img src='cid:embeddedImage' />), rather than referenced via a public url (i.e. <img src='http://www.mysite.com/myImage.png' />).
QUICK OVERVIEW - THE PUNCH LINE
I have written a CodeIgniter Library Extension to the basic Email class that facilitates the embedding of images in emails. You can download the code HERE.To implement it, follow the instructions for Extending Native Libraries on the CodeIgniter website http://codeigniter.com/user_guide/general/creating_libraries.html
Finally, in the body of your email, use the following macro - making sure the class_id attribute matches the one used in the img src attribute (i.e. <img src='cid:my_image' />). An exmaple message body would be as follows
<html>
<body>
<img src="cid:my_image" />
</body>
</html>
// Macro in Windows
{embedded_image file=C:\\my_image.png class_id=my_image}{/embedded_image}
//Alternative Linux Macro
{embedded_image file=/var/my_image.png class_id=my_image}{/embedded_image}
And thats it - the library encodes the image file as a base64 string and embeds it in your email.
PROS AND CONS OF EMBEDDING IMAGES IN EMAIL:
PROS
- Images are immediately displayed when opening a message, rather than the client prompting the user to allow remote content... great for newsletters- Emails are independent, once downloaded, they don't require a live connection to view in all their glory
CONS
- A little bit more coding and knowledge of email message format is required to get them to work if your framework does not support embedding images in email- Some conjecture about spam filtering for bulk messages with embedded images. Some believe spam filtering is more pessimistic for emails with embedded images than not when sending to more than 100 addresses
HOW TO EMBED AN IMAGE
I'm not going to go too far into the coding of native php code to email with embedded images (unless I get requests) - so Ill give a brief overview and some references.Like most things on the web, email message content can be defined in a series of envelopes. These envelopes are defined by content type. You can read about all of them here http://www.freesoft.org/CIE/RFC/1521/15.htm
One of the most useful images to exemplify the content structure of a html email can be found at http://www.phpeveryday.com/articles/PHP-Email-Using-Embedded-Images-in-HTML-Email-P113.html
Bit of a warning though - the article itself has buggy code and examples.
Grossly speaking - your email message if it supports html with embedded images, and a plain text alternative (for non html email clients) should render the following content in the email body
Content-Type: multipart/alternative; boundary="UNIQUE_ID_1"
--UNIQUE_ID_1
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
.... plain text alternative content for your html email....
--UNIQUE_ID_1
Content-Type: multipart/related;
boundary="UNIQUE_ID_2"
--UNIQUE_ID_2
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable
--UNIQUE_ID_2
Content-Type: image/jpeg
name='embedded_image.jpg'
Content-Transfer-Encoding: base64
Content-ID: <class_id_referenced_in_img_src>
Content-Disposition: inline;
filename='embedded_image.jpg'
(Base64 encoded binary for the image)
--UNIQUE_ID_2--
--UNIQUE_ID_1--
