The simplest approach would probably be to use <FilesMatch> to override the existing ErrorDocument and declare an image as the errordocument for missing images only:

ErrorDocument 404 /usual-error-document.html
#
<FilesMatch "\.(gif|jpg|png)$">
ErrorDocument 404 /404.gif
</FilesMatch>

Note three things:

1) I specified a .gif as the error image, because .gif filesizes are usually smaller. I did not specify a .png as the error image, because .png support is not universal. You could probably use .jpg successfully, but this rules out using a transparent image (see #3 below).

2) The URL-path to the errordocument (or in this case, error image) *must* be a local filepath. If you specify a full URL, then the response code returned by the server will not be 404, it will be 302-Found. This behaviour is documented in the Apache core ErrorDocument directive's documentation. This may mean that your replacment image will have to be served from "example.com" rather than from "images.example.com" and if so, then so be it.

3) The image that you provide as the error image must be designed to 'scale' to the aspect ratio and size specified in the HTML <img> tags of all of the images that it is to replace. Therefore, if it is *not* simply a 1x1 transparent .gif, then it must be kept *very* simple -- Perhaps an image of a couple of very-short lines of text in a very-plain font that will be readable even if distorted to fit a different aspect ratio and scaled up or down to fit a different size. I have had best success at replacing both "portrait" and "landscape" images by simply using a "square" replacement image which, on average, gets "stretched" the least in either dimension when replacing either of the other formats.

Jim

Many thanks, it works perfectly.