As I mentioned above, make a copy of archive.php and rename it to category-#.php
Look at the code in the newly made file and find the line that looks like <?php the_content(); ?> Not knowing your theme I do not know what it will be exactly and it may have more code before the ?>
Change this line to:
Code:
<?php if( get_post_meta($post->ID, "thumb", true) ): ?>
<a href="<?php the_permalink() ?>" rel="bookmark"><img class="thumb" style="float:left;margin:0px 10px 0px 0px;" src="<?php bloginfo('template_directory'); ?>/tools/timthumb.php?src=<?php echo get_post_meta($post->ID, "thumb", $single = true); ?>&h=<?php echo get_theme_mod('featured_top_left_thumb_height'); ?>&w=<?php echo get_theme_mod('featured_top_left_thumb_width'); ?>&zc=1" alt="<?php the_title(); ?>" /></a>
<?php else: ?>
<?php endif; ?>
<?php the_excerpt(); ?>
Bookmarks