You are probably referring to the image at en/Mainform.png relative to the main LOTRO folder. Shortly after opening the launcher, this image is replaced with advertising.
After a brief search, I did not find the advertising image in permanent local storage. It is possible that they push it through every time. If the image were found locally, it might be possible to replace it until a new advertisement were pushed through. IANAL, but this would surely be a violation of the EULA. Removing the image, if this means leaving an empty space, is the same.
Removing the image and resizing the form it is laid out on accordingly would require reverse engineering of the launcher which would be a grievous EULA violation as well.
It is a good guess that SSG would not include an option allowing their advertising to be disabled.
-nosplash and -skiprawdownload are the only useful launcher options I know.