Explicit Fixed Width and Height

add_filter( 'the_content', 'add_image_dimensions' );

function add_image_dimensions( $content ) {

    preg_match_all( '/<img[^>]+>/i', $content, $images);

    if (count($images) < 1)

        return $content;

    foreach ($images[0] as $image) {

        preg_match_all( '/(alt|title|src|width|class|id|height)=("[^"]*")/i', $image, $img );

        if ( !in_array( 'src', $img[1] ) )

            continue;

        if ( !in_array( 'width', $img[1] ) || !in_array( 'height', $img[1] ) ) {

            $src = $img[2][ array_search('src', $img[1]) ];

            $alt = in_array( 'alt', $img[1] ) ? ' alt=' . $img[2][ array_search('alt', $img[1]) ] : '';

            $title = in_array( 'title', $img[1] ) ? ' title=' . $img[2][ array_search('title', $img[1]) ] : '';

            $class = in_array( 'class', $img[1] ) ? ' class=' . $img[2][ array_search('class', $img[1]) ] : '';

            $id = in_array( 'id', $img[1] ) ? ' id=' . $img[2][ array_search('id', $img[1]) ] : '';

            list( $width, $height, $type, $attr ) = getimagesize( str_replace( "\"", "" , $src ) );

            $image_tag = sprintf( '<img src=%s%s%s%s%s width="%d" height="%d" />', $src, $alt, $title, $class, $id, $width, $height );

            $content = str_replace($image, $image_tag, $content);

        }

    }

    return $content;

}