ECC files are mainly associated with CryptoLocker or TeslaCrypt ransomware that encrypts and changes the extension of affected files to ecc. In case that you are affected by this, your best option is to restore from your backup, because you cannot really convert and restore these .ecc them back to their original format.
Except for the ransomware file, ECC format was some kind of very old graphics format used mainly for Manga and Anime cartoons. It is largely obsolete and no longer used, yet some programs still support it and may be able to convert these images to something more common, like .jpg pictures, which may be what some users are looking for when they search for ecc to jpg conversion.Open ECC file Open JPG file
A dedicated conversion software