ghilton,
For HTML email your best bet is to store the static image(s) on a server calling them from the email HTML in a absolute path. Mostly used email clients will remove embedded HTML images by default allowing the user to view image(s) from any email only if desired. For example we store all email images within a image directory on our webserver. Then we can call any image from any email and not raise the red flag.
Furthermore depending on the list you're sending to most providers, Comcast, sbc and others will watch port 25 for inbound/outbound loads and temporarily block large requests. So, if you can, check your email server for using and alternate SMTP port
