As I understand, you want the first image in your post to get inside your excerpt. So I have a better solution, why not make another function inside functions.php file that contains your image concatenated with the original excerpt?

Add to your functions.php

function get_new_excerpt(){
    $str = '';
    $img = '<img src="'.catch_that_image().'">';
    $linkedImage = '<a href="'.get_permalink().'" title="'.get_the_title().'">'.$img.'</a>';
    $str .= '<div class="post-image">'.$linkedImage.'</div>';
    $str .= '<div class="post-excerpt">'.get_the_excerpt().'</div>';
    return $str;
}
// Catch first Image from http://css-tricks.com/
function catch_that_image() {
  global $post, $posts;
  $first_img = '';
  ob_start();
  ob_end_clean();
  $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
  $first_img = $matches[1][0];

  if(empty($first_img)) {
    $first_img = "/path/to/default.png";
  }
  return $first_img;
}

then in your index.php or what ever page you want to show this function, insert:

<?php if(have_posts()) : while(have_posts()) : the_post(); ?>
       <div class="post">
            <h1 class="post-header"><?php the_title(); ?></h1>
            <div class="post-info">
                <?php echo get_new_excerpt(); ?>
            </div>
       </div>
<?php endwhile;endif;wp_reset_query(); ?>

you might want to add some css style like:

<style>
    .post {
        clear: both;
        background-color: #fff;
        border: 1px solid #ccc;
        overflow: hidden;
    }
    .post h1.post-header {
        border-bottom: 1px solid #ccc;
    }
    .post .post-info {
        clear: both;
        overflow: hidden;
    }
    .post .post-image {
        float: left;
        width: 100px;
        height: 100px;
        background-color: #EFEFEF;
    }
    .post .post-image img {
        width: 100%;
        height: 100%;
    }
    .post .post-excerpt {
        float: right;
        width: calc(100% - 120px);
        text-align: left;
    }
</style>