Hmm, I wonder if Wikipedia intentionally allows users to add HTML or if they just missed some input sanitizing. Or could it just be clever use of the content editor - maybe the editor allows you to specify image dimensions? Still not sure how they would get the fixed-position wrapper div in there, though, without typing in the HTML.

View source, line 795:

  <div style="position:fixed;left:0;top:0;"><a href="http://gnaa.press" rel="nofollow"><img alt="Blank space.png" src="//upload.wikimedia.org/wikipedia/commons/3/38/Blank_space.png" width="5000" height="5000" data-file-width="50" data-file-height="50" /></a></div>