The example url shows a scan with a size of 800px wide. The extension assumes that ANY size of an image will generate a thumbnail, but this is not the case. Thumbnails are only available for sizes smaller than the full size of the original image. So for this image, only thumbnail sizes smaller than 800px are available for usage.
The proofread extension should use the api to retrieve the proper thumbnail url, so that it can fall back to the actual image, in case a thumbnail url is not available.
Version: unspecified
Severity: blocker
URL: http://pl.wikisource.org/w/index.php?title=Strona:PL_Zamek_Kaniowski_(Seweryn_Goszczyński)_005.jpg&action=edit&redlink=&debug=true