Hey,
To cover your questions:
1.) Yes this is possible, open up category.php (this is the template your "preview section" aka category archive uses), and find this code:
<?php if (have_posts()) :
while (have_posts()) : the_post(); ?>
Immediately after it add this code:
<?php
// check if we got a previe picture, and which one should be taken
// (image resizing with "tim thumb" on? then we can take the big one and resize it)
$preview_small = get_post_meta($post->ID, "_preview_small", true);
$preview_medium = get_post_meta($post->ID, "_preview_medium", true);
$preview_big = get_post_meta($post->ID, "_preview_big", true);
//defaults:
$preview = $preview_medium;
$link_url = $preview_big;
$lightbox = 'singlepost';
$link = true;
//change if necessary:
// resizing? => take next sized picture
if ($k_option['general']['tim'] == "1" && $preview_medium == "")
{
$preview = $preview_big;
}
// no bigpicture? => no lightbox
if ($preview_big == "") { $lightbox = ''; $link = true; $link_url = get_permalink(); }
// the kriesi_build_image function used here checks if the image should be resized.
// the function is located in framework/helper_functions
$preview = kriesi_build_image(array('url'=>$preview,
'height'=> '273',
'width'=> '610',
'lightbox'=>$lightbox,
'link'=>$link,
'link_url'=>$link_url
));
?>
2.) You need to change this:
<?php
the_excerpt();
?>
to this:
<?php
echo $preview; // echo the preview image
?>
I agree with @Dude, the navigation is hard to find and use, consider moving it to the top of the page or at least adding the search box there.
Let me know if you have any other questions. :)