It's render by "ifsmain.css" from "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\LAYOUTS\INC"
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To determine whether the logo is turned on or whether the logo is turned off, run the following SQL script.
stsadm -o getformsserviceproperty -pn AllowBranding
To turn off the logo, run the following SQL script:
stsadm -o setformsserviceproperty -pn AllowBranding -pv false
Incase if you want it back, To turn on the logo, run the following SQL script:
stsadm -o setformsserviceproperty -pn AllowBranding -pv true