You need to modify archive.php to accomplish this. Open it up and just after this code:
while (have_posts()) : the_post();
add this code:
// 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'=> '250',
'width'=> '250',
'lightbox'=>$lightbox,
'link'=>$link,
'link_url'=>$link_url
));
You then need to add this code:
echo $preview; // echo the preview image
immediately before this code:
the_excerpt();
To adjust the image size, change the value of height and width (both currently 250).