No idea, never used twenty eleven theme. I opened the theme files. Check out showcase.php around Line 104-118.
Should be on line 115 – just remove the anchor wrapping the post thumb,
<a href="https://wordpress.stackexchange.com/questions/52498/<?php the_permalink(); ?>" title="<?php printf( esc_attr__( 'Permalink to %s', 'twentyeleven' ), the_title_attribute( 'echo=0' ) ); ?>" rel="bookmark"><?php the_post_thumbnail( $thumbnail_size ); ?></a>