Big Image on Featured Post, Normal on Single Post

You can show featured images at multiple sizes easily enough, by specifying the size of image when you make the call, but if you want them to be different you can use the code in this plugin:

http://wordpress.org/extend/plugins/multiple-featured-images/

Here’s a tutorial:

http://lifeonlars.com/wordpress/how-to-add-multiple-featured-images-in-wordpress/

e.g. Example registration:

if (class_exists('MultiPostThumbnails') {
    if (MultiPostThumbnails::has_post_thumbnail('folio', 'feature-image-2')) {
        MultiPostThumbnails::the_post_thumbnail('folio', 'feature-image-2'); 
    }
    if (MultiPostThumbnails::has_post_thumbnail('folio', 'feature-image-3')) {
        MultiPostThumbnails::the_post_thumbnail('folio', 'feature-image-3'); 
    }
    if (MultiPostThumbnails::has_post_thumbnail('folio', 'feature-image-3')) {
        MultiPostThumbnails::the_post_thumbnail('folio', 'feature-image-3'); 
    }
}

Example usage:

<div id="slider">          
    <?php    
        // Checks if post has a feature image, grabs the feature-image and outputs that along with thumbnail SRC as a REL attribute 
        if (has_post_thumbnail()) { // checks if post has a featured image and then outputs it.     
            $image_id = get_post_thumbnail_id ($post->ID );  
            $image_thumb_url = wp_get_attachment_image_src( $image_id,'small-thumb');                               
            $attr = array(
                'class' => "folio-sample",                                   
                'rel' => $image_thumb_url[0], // REL attribute is used to show thumbnails in the Nivo slider, can be skipped if you don't want thumbs or using other slider
            );
            the_post_thumbnail ('feature-image', $attr);                                            
        }
        if (class_exists('MultiPostThumbnails')) {                              
        // Loops through each feature image and grabs thumbnail URL
        $i=1;
            while ($i<=5) {
                $image_name="feature-image-".$i;  // sets image name as feature-image-1, feature-image-2 etc.
                if (MultiPostThumbnails::has_post_thumbnail('folio', $image_name)) { 
                    $image_id = MultiPostThumbnails::get_post_thumbnail_id( 'folio', $image_name, $post->ID );  // use the MultiPostThumbnails to get the image ID
                    $image_thumb_url = wp_get_attachment_image_src( $image_id,'small-thumb');  // define thumb src based on image ID
                    $image_feature_url = wp_get_attachment_image_src( $image_id,'feature-image' ); // define full size src based on image ID
                    $attr = array(
                        'class' => "folio-sample",      // set custom class
                        'rel' => $image_thumb_url[0],   // sets the url for the image thumbnails size
                        'src' => $image_feature_url[0], // sets the url for the full image size 
                    );                                                                                      
                    // Use wp_get_attachment_image instead of standard MultiPostThumbnails to be able to tweak attributes
                    $image = wp_get_attachment_image( $image_id, 'feature-image', false, $attr );                     
                    echo $image;
                }                           
                $i++;
            }                            

        }; // end if MultiPostThumbnails 
     ?>
</div><!-- end #slider -->