As you can see here, I have completed the second tile by accepting an answer to one of my questions. However, there is no indication of this accomplishment unless I physically mouse-over the tile and look at the text to the right:
Edit: this must be a bug. The actual sprite used in the CSS contains two rows of images - one light grey (as pictured above) and one blue.
Edit: I am using Firefox 16.0.2 on Windows 8 CP 64-bit.