Previous and Next posts are the single.php, not sure how you mean custom templates which are pages, how do they link to posts?
A link to the website and a scenario may help us understand!
There are limited scenarios for the single.php
This untested code may help, it assumes that the banner image is based on the author slug (nice name) and there is a default fallback image.
You already have a class in the style.css for the .author-banner, and on click it would take you to the authors (user) url for a splash page.
Example
The code would be in the single.php in the twenty ten theme just before
<div id="nav-above" class="navigation">
<?php
//Get the Author image by Digital Raindrops
function dr_author_image( $authorid ) {
$slug = the_author_meta('user_nicename', $authorid );
if ( file_exists( get_template_directory_uri() .'/images/'.$slug.'.png' ) ) {
return get_template_directory_uri() .'/images/'.$slug.'.png); ';
}else{
return get_template_directory_uri() .'/images/auth-banner.png); ';
}
}
?>
<?php if ( have_posts() ) while ( have_posts() ) : the_post(); ?>
<?php
//Get author image by and the author url
$style = 'background-image: url(' .dr_author_image( $post->post_author) .');';
$link = the_author_meta( 'user_url' );
?>
<?php //display the author banner and add a link ?>
<div class="author-banner" style="<?php echo $style; ?>" onclick="location.href='<?php echo $link; ?>'"></div>
The full twenty ten single.php someone else may want a look!
If the banner is a header full width banner then you could just do this in the header.php file, and use the conditional tag if( is_single() ) to only add the banner on the single.php pages.
HTH
David