logo disappear after update

Hi

I just created my custom theme section. It is possible to determinate the color background, the text color, the wrapper width and import a logo.

Everything works but when I update any of the color / width data, my logo disappear then from my frontend. So everytime I have to upload it.

How can I keep it ?

Here is my function for my logo uploading and settings validation:

function validate_setting($plugin_options) {
$keys = array_keys($_FILES);
$i = 0;
foreach ($_FILES as $image ) {
//if file was uploaded
if ($image ['size']) {
//is it an image ?
if (preg_match('/(jpg|jpeg|png|gif)$/i', $image['type']) ) {
$override = array ('test_form' => false);
$file = wp_handle_upload ($image, $override);
$plugin_options[$keys[$i]] = $file ['url'];
     }
	 else {
	 wp_die('No image was uploaded');
	 }
  }
  else {
  $options = get_option('plugin_option');
$plugin_options[$keys[$i]] = $options [$keys[$i]];
  }
$i++;
}
return $plugin_options;
}

Thanks for your help