For the facebook comments/share/etc I get the wrong thumbnail and as far as I've googled, it's due to the og:image meta tag. I'm using Drupal for Facebook module, is there a way to alter it? I found that I have to place 'rel="image_src"' somewhere, but where? At the image tag? And how can I do that using views?
The Metatags module supports OpenGraph meta data with token support, patterns, overrides for individual nodes and so on. It's a really handy module for this kind of stuff.
If you're getting the wrong image show up in Facebook you could point it to the correct image using this module's token support.
On the pages you like, alter the output of those pages like
There you can
and unset the previous og:image, or if you are in a node page you can load this node and display a field-image ...
This becomes handy when you are not on a node page, e.g. a Views page