As there was no help coming from this Forum, I embarked upon fixing myself and I did successfully. This is what I did.

/includes/functions/functions_lookups.php

I added following codes in the above file:

PHP Code:
/*
 * look up a products MEDIUM image and send back the image's HTML \<IMG...\> tag - ADDED BY GOPINATH GOSWAMI on June 25, 2015
 */
  
function zen_get_products_medium_image($product_id$width MEDIUM_IMAGE_WIDTH$height MEDIUM_IMAGE_HEIGHT) {
    global 
$db;

    
$sql "select p.products_image from " TABLE_PRODUCTS " p  where products_id='" . (int)$product_id "'";
    
$look_up $db->Execute($sql);

    return 
zen_image(DIR_WS_IMAGES $look_up->fields['products_image'], zen_get_products_name($product_id), $width$height);
  } 
Then, I edited the code in /includes/templates/YOUR-TEMPLATE/common/html_header.php as follows:

//get defined image or default
$myImage= zen_get_products_image((int)$_GET['products_id']);

was changed to

//get defined image or default
$myImage= zen_get_products_medium_image((int)$_GET['products_id']);

That's it... It worked like charm...!

I used facebook debug tool at https://developers.facebook.com/tools/debug/ and works perfectly.