e.g. I uploaded an image to http://caa.mbsandbox.org/release/0e6162b0-7780-4dc6-bc8f-b01366fb060a/cover-art
The cover art didn't work until the thumbnails were created

The image source was http://coverartarchive.org/release/0e6162b0-7780-4dc6-bc8f-b01366fb060a//808971955.jpg and loading that gave:

<?xml version='1.0' encoding='UTF-8'?>
<Error><Code>NoSuchBucket</Code><Message>The specified bucket does not exist.</Message><Resource>mbid-0e6162b0-7780-4dc6-bc8f-b01366fb060a not found by Metadata::get_obj()[server]</Resource><RequestId>2d26fc12-d603-4c0c-99c8-aba21d03c142</RequestId></Error>

I can reproduce this with nearly every image I upload. I managed to upload three images to a release before the images started to work. Once the images started to work, uploading further images worked fine.