As most email clients nowadays default disable pictures from external sources as a security measure, there's a need for a possibility to attach pictures inline the email itself.

Of course the best alternative would be as a "hidden" attachment, but even as a straight forward attachment will do - with such an attachment it would be possible to add the picture to the email with an <img src="cid:mylogo.jpg"/> - and they would be shown by default in the most of the email clients.

Best regards