They already have titles, which do much the same thing.
If you want alt tags too, that is a simple edit;
in /includes/functions/extra_functions/categories_dressing_functions.php, see the empty quotes '' in the first $cat_name_display= statement and replace with $cat_name_display
PHP Code:
function cat_name_display($cat_name_display$current_path) {
  
$cat_img_bg '';
  
//display image if exists for category name in current language - with title tag
  
if (file_exists(DIR_WS_TEMPLATE 'buttons/' $_SESSION['language'] . '/catimg' $current_path '.gif')) {
    
$cat_name_display zen_image(DIR_WS_TEMPLATE 'buttons/' $_SESSION['language'] . '/catimg' $current_path '.gif''''''''title="' $cat_name_display '"'); 
  
//display background image if exists for category name in current language - with title tag
  
} elseif (file_exists(DIR_WS_TEMPLATE 'buttons/' $_SESSION['language'] . '/catbg' $current_path '.gif')) {
    
$cat_name_display zen_image(DIR_WS_TEMPLATE_IMAGES 'pixel_trans.gif''''100%''100%''title="' $cat_name_display '"');
    
$cat_img_bg ' catBg' $current_path;
  } else {
    
$cat_img_bg '-text';//append to main classname
  
}
  return array(
$cat_img_bg$cat_name_display);

PHP Code:
    $cat_name_display zen_image(DIR_WS_TEMPLATE 'buttons/' $_SESSION['language'] . '/catimg' $current_path '.gif'$cat_name_display'''''title="' $cat_name_display '"'); 
The zen_image() function automatically adds the title when you give the alt value, so you can simplify the statement to
PHP Code:
    $cat_name_display zen_image(DIR_WS_TEMPLATE 'buttons/' $_SESSION['language'] . '/catimg' $current_path '.gif'$cat_name_display); 
Do the same for the second statement if you are using background images for cat names.