This is probably easy, but I can't figure it out. I've downloaded an HTML email template from http://www.campaignmonitor.com/templates/ which seems to just be an HTML file + images.
How do I use this with Thunderbird to send it as an HTML email?
See the instructions below to insert your HTML template:
When you do this just make sure that you have all your images hosted somewhere. (webserver, image hosting server. etc.) Sure the images may be visible while on your computer but when they are on someone else that will not be the case. They will see broken image placeholders.
Assuming this isn't just a SpamRoll, have you looked at their instructions? Where did you get stuck?