Looks like it does not work anymore.

By looking at network inspector, calls to google image api return:

/* callback */google.search.ImageSearch.RawCompletion('1', null, 403, 'This API is no longer available.', 200)

also breakpoint in setSearchCompleteCallback shows that imageResults array is always empty.

From the source it looks like the "feature" isn't implemented yet. Only image reference is:

$('#xkcd-image-'+(index)).html(" ");

This should be a "Show HN"