Kirbytag - check for image

I have a custom Kirbytext for YouTube Videos. However, this is my Kirbytag:

(youtube: 8386395 poster: my-poster.jpg)

How can I check in the Kirbytag if this Image exists at this page? I have tried so many but nothing seems to work. Also I’m not able to fully understand the code from the original image Kirbytag. My examples:

if ( $tag->poster->toFile() ) {
  $poster = 'FOUND';
} else {
  $poster = 'NOT FOUND';

Any simple solutions for this?

if ($tag->file($tag->poster)) {
  // do stuff

So simple … thanks a lot :smiley: