It's hard to tell from the picture, but it would appear the No. 2 spark plug has carbon deposits or ash buildup.
It would help if we could see the actual tips, where all the work is done.
Compare your plugs against the examples here:
http://roadstarclinic.com/content/view/70/127/ to get an idea of what could be your specific problem.
That said, from initial appearances I'd guess bad tube seals as well.