The ProofreadPage extension seems to be manually constructing thumb URLs and assuming it can slap in any old number. This fails when, for instance, we have disabled scaling of GIF images:
var proofreadPageThumbURL = "http://upload.wikimedia.org/wikisource/en/thumb/1/14/Summary_for_B-23.gif/WIDTHpx-Summary_for_B-23.gif";
Resulting files fail to render:
and so you're left with a big blank spot.