Auto-modifying original [full size] images

Yes, there is.

Found here:
http://www.wprecipes.com/how-to-automatically-use-resized-image-instead-of-originals

It will replace the original picture with the large size, defined in Media Settings (/wp-admin/options-media.php).

Here’s the code:

add_filter('wp_generate_attachment_metadata','replace_uploaded_image');

function replace_uploaded_image($image_data) 
{
    // if there is no large image : return
    if ( !isset($image_data['sizes']['large']) ) 
        return $image_data;

    // paths to the uploaded image and the large image
    $upload_dir              = wp_upload_dir();
    $uploaded_image_location = $upload_dir['basedir'] . "https://wordpress.stackexchange.com/" . $image_data['file'];
    $large_image_location    = $upload_dir['path'] . "https://wordpress.stackexchange.com/" . $image_data['sizes']['large']['file'];

    // delete the uploaded image
    unlink($uploaded_image_location);

    // rename the large image
    rename($large_image_location, $uploaded_image_location);

    // update image metadata and return them
    $image_data['width']  = $image_data['sizes']['large']['width'];
    $image_data['height'] = $image_data['sizes']['large']['height'];
    unset($image_data['sizes']['large']);

    return $image_data;
}

It is possible to use it with a custom size:

add_image_size( 'new-large', 1600, 1200 ); 

Changing all the occurrences of $image_data['sizes']['large'] in the original code with $image_data['sizes']['new-large']