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Since Drupal 7.20 was released, pdf_to_imagefield module doesn't properly render image derivatives.
The security fixes in this release change all image derivative URLs generated by Drupal to append a token as a query string. (As an example, links that previously pointed to a URL like http://example.com/sites/default/files/styles/thumbnail/public/field/ima... will now point to a URL like http://example.com/sites/default/files/styles/thumbnail/public/field/ima....)
I can verify this is the case. "Original image" works as expected, but setting an image style only shows a broken image icon.