@Roj
IANAL (and this isn't /. ;-)), but...
While the multicolored checkmark is obviously an Amiga logo, the letter A, checkered ball, and tilted red square -individually - are probably not copyrightable (but definitely trademarks). However, the specific combination of the four is unique to this site, and are probably protected as an unregistered copyright.
I've had several works of my own used by others, and in one specific case, by a competitor in direct competition with my own services. What can I do? Cry? Yell? Curse almighty God in heaven? I could, but that's about all the vindication I could scrape off. Otherwise, it's best just to accept it and move along.
What you could do is talk to a good copyright lawyer. As you are in the US, you should be aware that the US no longer requires an original work to be registered in order to be protected under copyright laws. All it requires is for it to be published. Again, no legal training here, but I would think putting something on a website would qualify as published.
