It would ease the load especially on the mysql version of the cluster code: when the browser sends an if-modified-since header, we can check for the image modification time (1 select), and, if has not been modified, skip selecting frome the image_data table.

Of course, the same applies to Oracle and PG.

Another interesting option would be the addition of an Etag http header made up of file modification time and size:
header('Etag: "'$row['SIZE'].'-'.$row['MTIME'].'"');

This is possibly easier to check later than the if-modfied-since header, which has a somewhat-variable-format date to be parsed.