Well, since it's not designed to do that out-of-the-box, you need to get creative.
So, first you find out "where" those images are being built. From browser View Source, you can find that they're in whatever page draws the indexCategories DIV.
So, searching the files for that reveals that it's in a template, which calls the tpl_category_row template. The first thing that template does is call the category_row.php module file in order to prepare the content.
Thus, it's in that module file that you would make your coding changes.
Since you want the images to be language-specific, you can probably do that by adding "de_" as a prefix to all the images applicable to the "de" language.
Thus, inserting the line of code as shown should accomplish giving the site the ability to first check to see whether an image exists with the "de_" prefix or not. If it does, it'll use it. If not, it'll default to using the image filename specified in the category details in the database, without any language prefix.
Be sure to save any changes you make into the appropriate override folder, rather than editing the original module file directly.
Code:
// $categories->fields['products_name'] = zen_get_products_name($categories->fields['products_id']);
if (file_exists(DIR_WS_IMAGES . $_SESSION['languages_code'] . '_' . $categories->fields['categories_image'])) $categories->fields['categories_image'] = $_SESSION['languages_code'] . '_' . $categories->fields['categories_image'];
$list_box_contents[$row][$col] = array('params' => 'class="categoryListBoxContents"' . ' ' . 'style="width:' . $col_width . '%;"',
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