Magento error “getimagesize(): Read error!”

Probably is explicit or maybe not, but this error is what happen when there is an a file under the media/import directory that as an invalid size, usually you can spot these files because they occupy 0 bytes.

Removing or uploading them again fix the issue.

How can I find the broken files?

With a simple bash command:
find . -size -1 -exec ls -l {} \;

Enjoy!