Hello,
updated here tatiana thema but now it doesn't show me that I have uploaded Has large image they uploaded.
Has small image
Has small image shows well.

@Karel
In file:
oc-content/themes/tatiana/admin/functions.php

Find:
.jpg
Change to:
.png

Hi
now no longer works, it just does not load images writes this error

Tam byla chyba při nahrávání velkého obrázku #661: extension not allowed, only allowed extension for large images is .jpg!

@Karel
This should be before first div tag:
<?php
  require_once 'functions.php';
  tatiana_backoffice_menu(__('Settings', 'tatiana'));
?>

<?php
// MANAGE IMAGES
if(Params::getParam('tatiana_images') == 'done') { 
  $upload_dir_small = osc_themes_path() . 'tatiana/images/small_cat/';
  $upload_dir_large = osc_themes_path() . 'tatiana/images/large_cat/';

  if (!file_exists($upload_dir_small)) { mkdir($upload_dir_small, 0777, true); }
  if (!file_exists($upload_dir_large)) { mkdir($upload_dir_large, 0777, true); }

  $count_real = 0;
  for ($i=1; $i<=1000; $i++) {
    if(isset($_POST['fa-icon' .$i])) {
      $fields['fields'] = array('s_icon' => Params::getParam('fa-icon' .$i));
      $fields['aFieldsDescription'] = array();
      Category::newInstance()->updateByPrimaryKey($fields, $i);
      message_ok(__('Font Awesome icon successfully saved for category' . ' <strong>#' . $i . '</strong>' ,'tatiana'));
    }

    if(isset($_FILES['small' .$i]) and $_FILES['small' .$i]['name'] <> ''){

      $file_ext   = strtolower(end(explode('.', $_FILES['small' .$i]['name'])));
      $file_name  = $i . '.' . $file_ext;
      $file_tmp   = $_FILES['small' .$i]['tmp_name'];
      $file_type  = $_FILES['small' .$i]['type'];   
      $extensions = array("png");

      if(in_array($file_ext,$extensions )=== false) {
        $errors = __('extension not allowed, only allowed extension is .png!','tatiana');
      } 

      if(empty($errors)==true){
        move_uploaded_file($file_tmp, $upload_dir_small.$file_name);
        message_ok(__('Small image #','tatiana') . $i . __(' uploaded successfully.','tatiana'));
        $count_real++;
      } else {
        message_error(__('There was error when uploading small image #','tatiana') . $i . ': ' .$errors);
      }
    }
  }

  $count_real = 0;
  for ($i=1; $i<=1000; $i++) {
    if(isset($_FILES['large' .$i]) and $_FILES['large' .$i]['name'] <> ''){
      $file_ext   = strtolower(end(explode('.', $_FILES['large' .$i]['name'])));
      $file_name  = $i . '.' . $file_ext;
      $file_tmp   = $_FILES['large' .$i]['tmp_name'];
      $file_type  = $_FILES['large' .$i]['type'];   
      $extensions = array("png");

      if(in_array($file_ext,$extensions )=== false) {
        $errors = __('extension not allowed, only allowed extension for large images is .png!','tatiana');
      }

      if(empty($errors)==true){
        move_uploaded_file($file_tmp, $upload_dir_large.$file_name);
        message_ok(__('Large image #','tatiana') . $i . __(' uploaded successfully.','tatiana'));
        $count_real++;
      } else {
        message_error(__('There was error when uploading large image #','tatiana') . $i . ': ' .$errors);
      }
    }
  }
}
?>




I guess I found it thanks

$extensions = array("jpg");

now it does not report any error but doesn't load the image

@Karel
.jpg to .png must be changed in oc-content/themes/tatiana/admin/settings.php, if you did not do it so.