Just preplace files in archive with yours.
Description what it makes:
Replace standart file controller/module/category.php to attach images from category to data array, also set size to 40x40, as you wish you can change it.
And replace view/theme/default/template/module/category.tpl only to add one line.
<img src="<?php echo $category['thumb']; ?>" alt="<?php echo $category['heading_title']; ?>" />
If you want you can move it inside link tag, and styling as you like.
Tested on 1.5.4 but should works on others.
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