They might be good as a speed test to blast through and to create the thumbnails. I don't know if this means overwriting the current files or if there is some trick to forcing the thumbnail recreation. If these work well, then I'll stop creating PNGs for the TIFF drawings I'm uploading (there are going to be something like another 20,000+ of these to come as part of HABS uploading).
Doing ?action=purge on an image description page will force all thumbnails to be re-rendered.
However I don't see much point in doing that. I'd much prefer people concentrate on doing new useful work instead of artificial "tests".
--bawolff