Add category image before post title on homepage

This is easily done using a filter on the title, which you can add to your functions.php. Like this:

add_filter ('the_title', 'wpse305812_add_flag', 10, 2);
function wpse305812_add_flag ($title, $id) {
  if (in_category ('england', $id)) $flag = "http://www.example.com/.../england.jpg"
  elseif (in_category ('brasil', $id)) $flag = "http://www.example.com/.../brasil.jpg"
  ...;
  return '<img src="' . $flag . '"> ' . $title;
  }

Note that this (fairly unelegant) code will not take into account the possibility of two flags assigned to one title.