There are a variety of ways to implement this. The simplest way would be to install Get the Image Plugin. The plugin automatically pulls the thumbnail of the first image uploaded per post and auto-adds a default image (which you set up) in case there is no image uploaded to that post.
After installing the plugin, open your search.php with text editor and just add and style a div for the image after the loop begins e.g.
<div class="results">
<div class="thumbs"><a href="<?php the_permalink() ?>" title="<?php the_title(); ?>"><?php if ( function_exists( 'get_the_image' ) ) { get_the_image('width=150&height=150&image_scan=true&default_image=https://yoursite.com/default.jpg' ); } ?></a></div>
<div class="blogposts">
<h2><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h2>
<?php the_time('F jS, Y') ?> by <?php the_author_posts_link(); ?><br />
<p class="postmetadata">Posted in <?php the_category(', ') ?> | <?php edit_post_link('Edit', '', ' | '); ?> <?php comments_popup_link('No Comments »', '1 Comment »', '% Comments »'); ?></p>
</div>
</div>
Then add something like this in your style.css:
.results { clear: both; margin-bottom: 20px;}
.thumbs { float: left; width: 150px; height: 150px; margin: 0 10px 10px 0; }
etc
edit- do not forget to upload the default.jpg (150px x 150px) to root folder and change the link yoursite.com to your site url.