I intend to send no more than one mailout a year.
You may need more than that to develop a relationship that results in money conversions. Have doubts? Ask your readership to register for a newsletter, and see if you get any bites.
HTML or plain text?
Embedded images or links?
All of the above. MailChimp can do this for you. You don't have to choose.
Sign up for the email newsletters of MailChimp and AWeber. Notice what they do. Read their stuff.
I've been learning about email, too. Here are a few things I've picked up --
25-35% do not have images enabled on html email. Those who do, often have it enabled on a case-by-case basis.
Using images in place of heading text may be simpler than figuring out how to get the text to display as desired with html. If you do that, use an alt that is the same as what the heading says.
Since the advent of tagged email programs like gmail, there is a tendency for archiving emails that can serve as reference material. Think of searched emails as access to a secondary Search engine. Use concrete language that will help the email come up in a search.
Including some sort of reference material in the email will help it get archived. It doesn't have to be unique -- eg "check out our review of the ten most important email marketing studies of the last year" could refer to a blog post written previously.
In the same spirit, archivers may be unlikely to do something at that moment, but may may want to join your FaceBook group or follow you on Twitter. Give them that opportunity, and stay in touch enough to be remembered.
Email doesn't equal spam. Some sorts of email lists have great clickthrus and reader adoption. A study I read a while back said that local events emails do really well. They have to be voluntary and they can't be spammy. Think of the people who get RSS subscriptions by email!
Where there is a subject and readership that merits frequent emails, the more you can send, the better. One tactic I've seen is to offer a multi-part introductory email series, with a tip a day or a tip a week, or a mini-course. They'd get a finite number of informational emails, with the option to choose another conversion - like a taste test before buying the item.