Image Upload from URL

you can write a php script, or make your own plugin of this code here, i used it in one of my projects where i had to import a large number of images.

first, get the image, and store it in your upload-directory:

$uploaddir = wp_upload_dir();
$uploadfile = $uploaddir['path'] . "https://wordpress.stackexchange.com/" . $filename;

$contents= file_get_contents('http://mydomain.com/folder/image.jpg');
$savefile = fopen($uploadfile, 'w');
fwrite($savefile, $contents);
fclose($savefile);

after that, we can insert the image into the media library:

$wp_filetype = wp_check_filetype(basename($filename), null );

$attachment = array(
    'post_mime_type' => $wp_filetype['type'],
    'post_title' => $filename,
    'post_content' => '',
    'post_status' => 'inherit'
);

$attach_id = wp_insert_attachment( $attachment, $uploadfile );

$imagenew = get_post( $attach_id );
$fullsizepath = get_attached_file( $imagenew->ID );
$attach_data = wp_generate_attachment_metadata( $attach_id, $fullsizepath );
wp_update_attachment_metadata( $attach_id, $attach_data );

and voila – here we go.
you can also set various other parameters in the attachment array.
if you got an array of urls or something like that, you can run the script in a loop – but be aware that the image functions take up a lot of time and memory to execute.