You can put this into Style & edit CENTER COLUMN -> Content ABOVE the LOOP
PHP Code:
<?php if ( is_category() ) { ?>
<img src="/path/to/images/<?php echo $wp_query->get_queried_object_id(); ?>.gif" alt="" />
<?php } ?>
And in /path/to/images/ upload a
[ID-of-category].gif for each category, i.e. 12.gif, 17.gif, 21.gif