You may try to increase the filter params via the execution of following MySQL query:

update `sys_transcoder_filters` SET `filter_params`='a:2:{s:1:"w";s:4:"1920";s:1:"h";s:3:"960";}' where `transcoder_object`='bx_persons_cover' and `filter`='Resize' limit 1;

Then need to clear the DB cache via Studio->Dashboard