As I understand it, this API call effectively creates a campaign based on a fixed template created on the fly from the HTML input gotten from the URL parm value. When it grabs the input from the HTML URL, does it capture a copy of images and style sheets to localise everything?
Ah, Google just found me this link http://premailer.dialect.ca/ and it looks like CM uses and sponsors this program, so I think this answers my question for the CSS. How about the images though?
Yes, when Campaign Monitor imports from a URL all the images will be imported and served on our end, and the CSS will be grabbed and placed inline automatically for you.
You can selectively prevent image importing using the cm_dontimportimage attribute on any image. See http://help.campaignmonitor.com/topic.aspx?t=83