Missing alt-attributes

Hi,

on our site we are using the Visual Composer “Logo Grid” element.
Unfortunately the “alt” attributes are not provided there.

// Image output
$image_output = '<img src="'. esc_url($att_img_url[0]) .'" alt=""/>';

I have tried to override the file (boc_logo_gallery.php) in the child theme but it’s not working.
What have to be considered in such a process? Is it even possible?

Thanks for your help!