Photo.png file hacked or malware. How do I replace essential site file?

wp-admin/images/test.png: PHP-FILEHACKER-yy.UNOFFICIAL FOUND

Hi,
A test by my web host provider and also Wordfence identifies the file (above) as bad. However if I move this file to the trash, my site does not work, it seems it is essential.

Is there a way to obtain a new file, if this one is faulty or has been tampered with?

Thanks