I found a solution to this by adding the images to the body and not in the meta information, then just hiding them, like this;
<body>
...
<img src="http://image1" width="41" height="41" border="0" alt="text describing the image" style="visibility:hidden" /><br/>
<img src="http://image2" width="41" height="41" border="0" alt="text describing the image" style="visibility:hidden" /><br/>
etc
...
</body>
It just came to me from reading that if no og:image property is given then thumbnails are taken from the body, and it worked even if hidden. You could probably remove the width, heigh etc to tidy it up, but I thought why both waste time with that when it gets hidden anyway, I just cut and paste and made it hidden.
Hope this helps.
