Im having the same problem and havent solved it yet .
If I post my site on facebook i can choose images from the slideshow module.
To prevent the slideshow images to be used as thumbnail for facebook this is what i did:
I edited head.php in /templates/mytemplatename/blocks
and added this line
<link rel="image_src" href="http://imagesource/myimage.jpg" />
before this line
<jdoc:include type="head" />
I then used the facebook debugger http://developers.facebook.com/tools/debug
and in the displayed results the preview is visible and pictures are no more pulled out of the slideshow as it was before
But when I post my link on facebook i still have the slideshow pictures to choose a thumbnail from and NOT my defined preview picture.
Has anyone fixed this ?
Any hints on how to fix this ?
Thx in advance