Previously it was suggested here that it was the CSS hack that was causing the problem, and to some extent that is true.
Viewed with Firefox or Safari, the Google cache of the relevant pages does not show the images on Variations 3 and 4.
However, this was previously been taken as indicating that the images had not been cached by Google. It now seems veryunlikely that this is the case, because the images render perfectly well when looking at the cache via Internet Explorer.
In other words, the experiment was a bit misconceived. It would have been possible to rule out the underlying hypothesis by simply checking cached pages in various different browsers.