It is in fact just a modification of Gmail to Google Mail logo by apexad@userstyles (credits to him) where I replaced Google Mail logo with clean Gmail logo. Advantage of this style to other similar is that it works in print window too!

If you want to put your own image in place of Gmail logo, make 143x59 gif, encode it as MIME (Base64), and put code in here:
url("data:image/gif;base64,PUT_YOUR_GIF'S_B64_CODE_HERE")

For example: (example removed)

Image is never downloaded, but decoded from style!


UPDATE:
Just tried it out. Clean gmail logo is showed on print page, but print preview or printed mails are with no logo at all! If anyone knows how to achieve to keep it on printed mails, mail me to banzek@yahoo.com or leave a comment and I'll update the style.
If you want to clean printed mail you can use Gmail - Remove logo on printer friendly page by shigefumi@userstyles.