Use a Default Category Post Thumbnail

<!-- Conditional thumbnail for categories -->
<div class="excerpt-thumb">
    <?php if (  (function_exists('has_post_thumbnail')) && (has_post_thumbnail())) : ?>
        <a href="<?php the_permalink(); ?>" title="<?php printf( esc_attr__( 'Permalink to %s', 'twentytwelve' ), the_title_attribute( 'echo=0' ) ); ?>" rel="bookmark">
        <?php the_post_thumbnail('excerpt-thumbnail', 'class=alignleft'); ?>
        </a>
    <?php else :?>
    <a href="<?php the_permalink(); ?>" title="<?php printf( esc_attr__( 'Permalink to %s', 'twentytwelve' ), the_title_attribute( 'echo=0' ) ); ?>" rel="bookmark">
        <img class="alignright" src="<?php echo get_stylesheet_directory_uri(); ?>/images/<?php $category = get_the_category(); echo $category[0]->slug; ?>.png" />

    <?php endif;?>
</div>