If you predict many subscribers then going 3rd party is the only way, and by 3rd party I mean companies that specialize in bulk mail sending and have an API to control it. The API should satisfy your need for fine control. WordPress.com and feedburner are not the kind of services I’m thinking of here.
You can use WP for newsletters as long as you send text only mail (formatted as text or html), but once you send images you need to start dealing with traffic picks you are not used to when running your site. If you embed the image in the mail you will basically send it to each of your subscribers which will result in outgoing traffic surge which might annoy your upstream provider. If you leave it on your server and just refer to it with an IMG tag in your mail, then your server might get hammered by receiving tens of request for the image at virtually the same time.
The ultimate solution in my opinion is to do the user registration on WP side, and use experts to handle the actual transmission.
But then, if you are going to have only tens of subscribers then you can probably DIY without getting into real problem as long as you handle the spam related stuff correctly.