Modify loop output for a specific list of categories

I'm trying to modify the output of a loop on an archive page for a specific list of categories. Normally this loop outputs the Title, Author, Date and does so for every post in that given category.

I have a list of categories in which I'd like to modify that output to include images. Up until now I've accomplished this by using if statements like below:

class new_loop extends theme_custom_loop {

 function modify_categories() {

  $cat1 = '101';
  $cat2 = '201';

  if (is_category($cat1)) {
    echo 'stuff';
  }

  elseif (is_category($cat2)) {
    echo 'stuff';
  }

  else theme_loop();

 }

}

Now I have a list of about 40 categories. I've tried to store them in an array and run them through a loop but it doesn't seem to work. Any advice? Current code is:

class new_loop extends theme_custom_loop {

  function modify_categories() {

    $archive_categories = array(
         '101',
         '201'
     );

     foreach($archive_categories as $archive_category) {
        if (is_category($archive_category)) {
            echo 'stuff';
        }

        else theme_loop();
     }
  }
}