Filter a pluggable function

As noted in @Milo’s comment, a function being “Pluggable” is entirely separate from it being filterable. A Pluggable function can be overridden wholesale, because of the if ( function_exists() ) conditional in which it’s wrapped. A Plugin is filterable if its output is parsed through an apply_filters() call.

Since you only want to change a couple values in the returned array, you want to add a filter, rather than Plug the entire function.

Simply add the following in your Child Theme functions file:

function wpse117251_wpsight_layout_images( $layout_images ) {
    // Modify array values
    $layout_images['size_archive_listings'] = 'your-string-here';
    $layout_images['align_archive_listings'] = 'your-string-here';
    // Return the array
    return $layout_images;
}
add_filter( 'wpsight_layout_images', 'wpse117251_wpsight_layout_images' );