A Chilkat customer pointed me to this useful information:
http://www.campaignmonitor.com/css/
Important: I don’t know if the writers at campaignmonitor.com are aware of the different ways embedded images may be referenced (from within the CSS or from within HTML). It can be via “CID” or by URL w/ matching Content-Location header field. Chilkat can do both by setting the UseCID property (true/false). It may be that in some cases “CID” does not work, but URL/Content-Location does, or perhaps the reverse. This would need to be studied in more detail…