Sorry I don't understand your answer.
You asked me to raise this as a feature request (which I thought I had!) after I raised this issue originally as a support ticket.
I don't know how the thumbnail image is implemented, all I know is that if there is a personalised image in the new order email it is blocked by gmail. If there are two or more personalised images in the new order email then it really messes up the output and shows lots of strange characters going on for pages and pages, making the email unreadable! This email is also sent to the customer! I don't want to switch the thumbnail image off because all other customers not using gmail to read their emails will see the thumbnail images fine.
Actually we are not using data uris anymore for images in the mail, we create real small thumbnails and embed these in the mail. The data uri is only used as fallback when something went wrong during that image creation process.