Just don't do it. We recently moved to Outlook and almost immediately some of our users found that they could put backgrounds on their email messages. One user actually started using a background that made the email text itself difficult to read. I don't know if someone said something to the user about it, but she finally did drop the background. I don't know what it does to the message size, but I think that backgrounds on emails are just plain annoying. Back when I was doing helpdesk work, I would go out of my way to strip annoying backgrounds and text styles out of emails before replying to them.