Files can be attached to an e-mail using the createAttachment() method. The default behavior of Zend_Mail is to assume the attachment is a binary object (application/octet-stream), that it should be transferred with base64 encoding, and that it is handled as an attachment. These assumptions can be overridden by passing more parameters to createAttachment():

$mail = new Zend_Mail();
// build message...
$mail->createAttachment($someBinaryString);
$mail->createAttachment($myImage,
                        'image/gif',
                        Zend_Mime::DISPOSITION_INLINE,
                        Zend_Mime::ENCODING_BASE64);

If you want more control over the MIME part generated for this attachment you can use the return value of createAttachment() to modify its attributes. The createAttachment() method returns a Zend_Mime_Part object:

$mail = new Zend_Mail();

$at = $mail->createAttachment($myImage);
$at->type        = 'image/gif';
$at->disposition = Zend_Mime::DISPOSITION_INLINE;
$at->encoding    = Zend_Mime::ENCODING_BASE64;
$at->filename    = 'test.gif';

$mail->send();

An alternative is to create an instance of Zend_Mime_Part and add it with addAttachment():

$mail = new Zend_Mail();

$at = new Zend_Mime_Part($myImage);
$at->type        = 'image/gif';
$at->disposition = Zend_Mime::DISPOSITION_INLINE;
$at->encoding    = Zend_Mime::ENCODING_BASE64;
$at->filename    = 'test.gif';

$mail->addAttachment($at);

$mail->send();