I have a lot of images on my site and had no trouble until this week when I added some more. For no good reason I received an error message during one upload (unfortunately not noted). I edited that node and re-uploaded the image - which went OK. Since then has been hell, with new galleries being fine and thumbnails displaying correctly, followed by all the thumbnails disappearing minutes later and gallery pages displaying fullsize versions instead - totally breaking the layout.
Searching on Drupal revealed this is not an uncommon problem. Various suggestions such as using smaller originals, did not make any difference here. Nor does the mod to image module code at http://drupal.org/node/71178#comment-147675
From looking at the files table all records with nid's higher than the one that gave the initial upload error are prone to losing all versons (thumbnail and preview) except the original file. After re-uploading all records are present. Sometme later, only the original file records remain.
After many hours experimenting I have now found that it's cron.php which is causing the thumbnail and preview records to be vaped. So long as I don't run it, everything is fine. As soon as I do, they're gone.
I have exported the files table as sql and cannot see anything wrong with the sql at all. If I drop the table and import the sql I get all versions back again until cron.php runs. This is repeatable and 100% definite now.