Changing Image Size on Home Page

Hi all,
I am looking for a little bit of friendly guidance in altering the size of the thumbnail images on the homepage of my blog https://londoncows-blog.co.uk/. I would like to make them a little bit bigger, and having found out what I thought to be the answer, managed to completely mess up. I think I need to alter the index.php file, and here is the code, but as mentioned I couldn’t get it to work out for me:

<?php get_header(); ?>
<?php
	$options = 

get_option("widget_sideFeature");
	$posts = get_option('fms_number_posts');
    $paged = (get_query_var('paged')) ? 

get_query_var('paged') : 1;
	if (is_active_widget('widget_myFeature')) {
		$category = 

"showposts=".$posts."&cat=-".$options['category']."

&paged=".$paged;
	} else {
		$category = 

"showposts=".$posts."&paged=".$paged;
	}
	$i = 1;
	?>

	<?php query_posts($category); ?>
	<div id="threecol"><div id="threecol2">
	<?php while (have_posts()) : the_post(); ?>
		<?php if($i == 7) { $i=4; } ?>
		<div class="threepost 

threepost<?php echo $i; $i++; ?>">
			<h2><a href="<?php 

the_permalink() ?>"><?php the_title(); ?></a></h2>
			<div class="storycontent">
				<?php 

getImage('1'); ?>
				<?php echo 

theme_excerpt(25); ?>
				<p 

class="contread"><a href="<?php the_permalink(); 

?>">Read More &raquo;</a></p>
			 </div>
		 </div>
	<?php endwhile; ?>
		</div>
	</div>
    <div class="navigation">
        <div class="alignleft"><?php 

next_posts_link('&laquo; Older Entries') ?></div>

<?php include (ABSPATH . 

'/wp-content/plugins/featured-content-gallery/galle

ry.php'); ?>

        <div class="alignright"><?php 

previous_posts_link('Newer Entries &raquo;') 

?></div>
    </div>
<?php get_footer(); ?>

Any help would be much appreciated. Nearly forgot to mention the theme is Feed Me Seymour.