pagelist thumbnail image from a block (no attribute image) ?

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how can I get a thumbnail from the image of an custom block.

in the blog_index_thumbnail template I found this :

$ts = $cobj->getBlocks('MyCustomBlock');
      if (is_object($ts[0])) {
         $tsb = $ts[0]->getInstance();
         $thumb = $tsb->getFileObject();
         if($thumb){
         $imgHelper->outputThumbnail($thumb, 220, 220, $title);
         }
      }
      $imgHelper->outputThumbnail($thumb, 220, 220, $title);
      }


OK I can add my Block name, but where and how did I define my image $MainImage that I will show as thumbnail?

Any Ideas?