Linden Labs is not using it on the grid. I'm talking about them using it here on the forum. This is a separate entity from their in-game server systems. If they use the image that was posted here by another user on this forum despite me denying them permission to do so, then yes, they are liable to have action taken against them. So long as they don't do that, they are fine.
I've already reported the theft of the image to their forum moderators for removal. If they refuse, then yes, a DMCA take-down notice will be warranted, but I'm hoping it doesn't get that far. My comment about me suing Second Life for using it was directed at the person who thought it was funny to say that they should use it on the forum anyway despite me not granting them permission for such. Second Life, (and Linden Labs), are safe from that as long as they don't use it on their forum.