Hi Isomerville,
On line 304 of ‘import-external-images.php’ change this :
if ( $uri != '' && preg_match( '/^http:\/\//' , $uri ) ) {
to this :
if ( $uri != '' && preg_match( '/^https?:\/\//' , $uri ) ) {
The problem is that your image is on https, but the regex was only trying to match with http.