Facebook sharer, thumb is showed instead of large image - php

Question

I have set up a contest, the contestants are asked to share their results on facebook.
I have the following button to share the results:
print('<a class="facebook stemspeler" style="color:#eec920; margin-right:15px;" target="_blank" onclick="return !window.open(this.href, \'Facebook\', \'width=1200,height=300\')" href="https://www.facebook.com/sharer/sharer.php?u=http%3A%2F%2Fwww.***.com%2Fallstars&picture=http%3A%2F%2Fwww.***.com%2Ftypo.php?id='.$makeUniq.'&title=tet&caption=test&quote=test&description=test">‌​Share</a>');
The $makeUniq makes sure that facebook fetches the results instead of caching the results the first time (When link stays the same it caches, now it doesn't because link changes every request).
The typo.php is the file that houses the image. It takes a standard template and transforms the contest inputs to a image overlay.
<?php
session_start();
header("Cache-Control: no-store, no-cache, must-revalidate, max-age=0");
header("Cache-Control: post-check=0, pre-check=0", false);
header("Pragma: no-cache");
// Output to browser
header('Content-Type: image/png');
// Create a 300x150 image
//WHITE BACKGROUND COLOR
$im = imagecreatetruecolor(1000, 1000);
$im = imagecreatefrompng("images/test.png");
$black = imagecolorallocate($im, 0, 0, 0);
$white = imagecolorallocate($im, 255, 255, 255);
$test = imagecolorallocate($im, 255, 255, 0);
// Set the background to be white
imagefilledrectangle($im, 0, 0, 900, 990, $test);
// Path to our font file
$font = 'fonts/DINPro-Medium.ttf';
$fontSize = 24;
$angle = 0;
$x = 80;
$y = 150;
// Write it
imagettftext($im, $fontSize, $angle, $x, $y, $white, $font, ' : n');
$x = 80;
$y = 200;
// Write it
imagettftext($im, $fontSize, $angle, $x, $y, $white, $font, 'k : ');
$x = 80;
$y = 250;
// Write it
imagettftext($im, $fontSize, $angle, $x, $y, $white, $font, 'Ler :n');
$x = 80;
$y = 300;
// Write it
imagettftext($im, $fontSize, $angle, $x, $y, $white, $font, 'Mouwer : ');
$x = 80;
$y = 350;
// Write it
imagettftext($im, $fontSize, $angle, $x, $y, $white, $font, 'Rwer n');
$x = 80;
$y = 400;
// Write it
imagettftext($im, $fontSize, $angle, $x, $y, $white, $font, 'Rechtk : ');
$x = 80;
$y = 450;
// Write it
imagettftext($im, $fontSize, $angle, $x, $y, $white, $font, 'Cir : ');
imagepng($im);
imagedestroy($im);
?>
Up till here everything works like its supposed to be.
The problem is that when the facebook share functionality opens, facebook keeps showing the image as thumbnail instead of large image.
And the result I'm aiming for is the large image:
I have tried to change resolution many times, and there are several posts on the internet that are close to describe this problem.
Anybody has any idea how I could fix this problem?
Thanks in advance!

Answer 1

You can try to specify these OG elements:
og:image:width
og:image:height
these tags helps crawlers to obtain you image size before dowload it. Maybe you're getting a smaller image preview because crawler didn't get the right size at first and now you're getting preview from a cache.
EDIT:
Of course you should be sure that you're using an image that is at least 1200 x 630 px

Answer 2

The recommended picture size for a shared link on Facebook is 1200x630. If the picture is too small, Facebook will center crop it and only show a thumbnail of the picture. I believe de minimum size is 600x315, but the result might not be as good on high resolution devices. If you want it to look good and show the full size picture on all devices, use 1200x630px pictures.
You can find a references to those best practices
here.
EDIT:
Your problem might be due to the fact that the Facebook crawler caches the info it crawls, so your old picture might still be used by FB even if you fixed its resolution. Make sure to go here, to force the Facebook crawler to re-crawl you website and get the new data or simply empty its cache for a particular URL to force it to recrawl your website next time you share the link. Otherwise FB probably still use you old picture it already has in its cache.