I want to have a background image while I compose a new message in Gmail. I must be able to type over it. Is it possible?
From Gmail help topic: http://mail.google.com/support/bin/answer.py?hl=en&answer=8260
Rich formatting is not available in Gmail's basic HTML view or in your signature settings. At this time there is no way to save customized formatting options as a default Gmail setting.
There is no way to do that with their editor, so no way to make it simple.
However, since such thing is most likely linked to an HTML mail, you can probably do it yourself by writing the email in HTML and putting the background yourself. This is beyond my knowledge, though, and someone else would have to explain how.
Another possibility would be to use Gmail as any mail provider, and send emails from a program like Outlook or Thunderbird, and define background there. But I guess it misses a bit the point, there.
This may fly in the face of how you want to do it but, you can set up your Gmail account as a regular email (have to enable POP/IMAP in gmail.com prefs first) Once you've done that .. I use Mozilla's SeaMonkey (think Firefox with an email client) and when composing my message go to Tools, format page colors and background then select your background image pick colors that will show up clearly over it and click ok.
For me, this is the easiest way to put a background in my messages. I only use a background once in a blue moon but, when you want to, well .. go for it. (sorry it's not an answer that will work in the web)