How to set different default images in an image field per domain?
Point the image to a .php file instead of a .jpg file, and then write a script to


Find the domain name (https://stackoverflow.com/a/17201254/836995)
Output the appropriate image (https://stackoverflow.com/a/1353867/836995)


Something like this should work (untested):

&lt;?php

$img_file = NULL;

switch ($_SERVER['SERVER_NAME']) {
  case "www.a.com":
    $img_file = "filename_a.jpg";
    break;
  case "www.b.com":
    $img_file = "filename_b.jpg";
    break;
  default:
    break;
}

if (!empty($img_file)) {
  header("Content-type: image/jpeg");
  readfile($img_file);
}


This is a bare minimum script. A more elaborate version here: https://stackoverflow.com/a/9453207/836995 (although you don't need the .htaccess part to do this for only one image).