yeah, I've been thinking of doing this. Haven't got around to it yet, but my guess is that you'll need to add a new image size setting in the back office, and then change the section below in category.tpl to load the new image size:
<div class="subcategory-image">
<a href="{$link->getCategoryLink($subcategory.id_category, $subcategory.link_rewrite)|escape:'html':'UTF-8'}" title="{$subcategory.name|escape:'html':'UTF-8'}" class="img">
{if $subcategory.id_image}
<img class="replace-2x" src="{$link->getCatImageLink($subcategory.link_rewrite, $subcategory.id_image, 'medium_default')|escape:'html':'UTF-8'}" alt="" width="{$mediumSize.width}" height="{$mediumSize.height}" />
{else}
<img class="replace-2x" src="{$img_cat_dir}default-medium_default.jpg" alt="" width="{$mediumSize.width}" height="{$mediumSize.height}" />
{/if}
</a>
</div>