Problem changing image pls help

Hi

I have just installed a new template which has an image on the index page of a bridge at night (check it out:https://site-hero.com) The thing is I want to replace this image with a flash animation I have but I am a complete novice with code, I can just about understand what I’m reading but editing is a big No No for me. I already have the WP-SWFObject installed on the blog as you can see I have managed to put the animation in a post. So can somebody tell me (in childlike simplicity) how I would change this image. This is the code I believe I need to put in somewhere but I don’t know where:
<?php wp_swfobject_echo(“MY ANIMATION URL.swf”, “400”, “280”); ?>

Also to help here is the code I have cut from the main index template (the file name for the image that I want to change is “mainimage”):
<?php get_header(); ?>

<div id=”content”>
<div id=”contentleft”>

<div class=”blogpostwrapper”>
<div id=”mainimage”>
<img src=”<?php bloginfo(‘template_directory’); ?>/images/mainimage.jpg” class=”mainimage” alt=”Main image” title=”Main image” />
</div>

<?php if (have_posts()) : ?>

<?php while (have_posts()) : the_post(); ?>

<div class=”blogpost”>
<div class=”blogtitle”>
<h2>” rel=”bookmark” title=”Permanent Link to <?php the_title_attribute(); ?>”><?php the_title(); ?></h2>
</div>
<div class=”datetab”>
<div class=”blogdate”>
<?php the_time(‘jS F’) ?>
</div>
<div class=”blogdateyear”>
<?php the_time(‘Y’) ?>
</div>
</div>
<div class=”blogauthor”>
written by <? the_author(); ?>
</div>
<div class=”blogcontent”>
<?php the_content(); ?>
<div class=”blogtags”>
<?php the_tags( ‘Tags: ‘, ‘, ‘, ”); ?>
</div>
</div>
<div class=”blogdivide”>
</div>
<div class=”blogstrip”>
<div class=”blogcomments”>
<?php comments_popup_link(‘0 Comments’, ‘1 Comment’, ‘% Comments’); ?>
</div>
<div class=”blogcategory”>
<?php the_category(‘, ‘) ?>
</div>
</div>
</div>
<?php endwhile; ?>
<div id=”paging”>
<?php $previous = get_bloginfo(‘template_directory’); ?>
<div id=”alignright”><?php next_posts_link(‘<img src=”‘ . $previous . ‘/images/previous.png” alt=”Previous” title=”Previous” />’) ?></div>
<div id=”alignleft”><?php previous_posts_link(‘<img src=”‘ . $previous . ‘/images/next.png” alt=”Next” title=”Next” />’) ?></div>
</div>
<?php else : ?>
<h2 class=”center”>Not Found</h2>
<p class=”center”>Sorry, but you are looking for something that isn’t here.</p>
<?php include (TEMPLATEPATH . “/searchform.php”); ?>
<?php endif; ?>
</div>
</div>
<?php get_sidebar(); ?>
</div>
<?php get_footer(); ?>

Sorry this is such a LOOOOONGG post Hope someone can help me.

Rich