Invalid Argument

The assumption here is that you are using WordPress Version 3.0.1.

I don’t know why it suddenly stopped working, but you should check lines 124 to 128 of /wp-admin/includes/image.php and be sure they read exactly as follows:

foreach ($sizes as $size => $size_data ) {
            $resized = image_make_intermediate_size( $file, $size_data['width'], $size_data['height'], $size_data['crop'] );
            if ( $resized )
                $metadata['sizes'][$size] = $resized;
        }

If you don’t have any way to figure out line numbers, NotePad++ is a free editor that shows line numbers. https://notepad-plus-plus.org/

This code is part of a function that is creating information about the thumbnail that has been generated of your picture.