I just want to say: Thank you very much for this really helping advices!! It’s nice to hear from someone who wants to helps people like me who are just begining in the illustration field.

My advice would be to keep your emails simple with an obvious link to your portfolio. The thing with attaching images or using HTML is that your email may be diverted to the recipient’s spam folder, depending on what their privacy settings are. If you don’t bury a link to your site in a lot of text, anyone who is interested is only a click away from seeing your work.

If however, you are sending promo emails to people you have contacted before or who have signed up to your mailing list, you are more likely to get images and HTML past their spam blockers, since you are sending from a recognized address.

Thanks for the question, Nate. I hope this helps.
Thomas