Why does Vk share not catch PNG image format?

Question

JavaScript

I use this code:

<script type="text/javascript" src="http://vk.com/js/api/share.js?90" charset="windows-1251"></script>

<script type="text/javascript"><!--
document.write(VK.Share.button(false,{type: "custom", text: "вконтакте"}));
--></script>

when you click the "share" button, the png image does not get into the ball, and the rest of the posts with jpg get. What's the problem, I wonder?

Answer 1

For the correct balls in VK, in the head tag I indicate a link to a photo of a post of any extension:
<link rel="image_src" href="/img/poster.png" />