If you have an HTML email with "spacer.gif" repeated several times, then you get
that image included several times in the payload of the email. An optimization
would be to "share" the sid across the several references.
I have adapted HTMLEmail for my own purposes, and one of the "fixes" I have
added is just such an optimization.
Please could a Committer register their interest in this, and I will make a
patch and submit for your consideration.
as a separate class HtmlEmail2 which is attached to that bug report.
Within that class is my implementation of a fix to this issue.
That code is cannot be patched directly against HtmlEmail, but I could provide a
patch if there is interest in my doing so.