When you have a robust list of free registered users, you should routinely try to upsell them into a premium membership.
Marketing Profs does this nicely with this email. The image gives a witty take on the upsell, making it more engaging than a boring offer. The use of a code encourages word-of-mouth marketing (as opposed to offers made strictly to registered users).
Also, the email contains a link for a free trial, but it’s subtle (“Sample some PRO content”) thus making sure that the emphasis is on the actual sale, not more lead generation techniques.
Lastly, the headshot and phone number are great relationship-building items. And the PS mentioning an expiration date on the offer is a great way to give a sense of urgency so prospects “buy now!”