So we have a client who is sending emails out using CID embedding (sub-optimal, I know, but...clients) - seems to be working for them except for a background image that is created using the tool at http://backgrounds.cm/. They're reporting some issues in Outlook and potentially other clients.

I guess my question is: is there anything against using CID images as background images? either in the <td background=""> or in CSS?

Sorry for being vague - mostly just looking for a place to start debugging for them.