Ok all.
I cracked it.
Recently I made a new website for a customer and hit the same issue with facebook like button not showing images from the post/pages. I spent a few nights and found the sollution.
Problem is this: Not from long time ago, facebook changed the way like button works. In order to have a facebook like button on your webpage you need to make an API on the developer section of facebook.
This is what i made:
1. I installed Facebook AWD All in One plugin. A great plugin for managing all facebook utilites. (Like Button, Like Box, Comments, Login Button)
2. After installing, the plugin requires setting up. You will be asked to insert a API id, Admin id, Page Admin id.
3. Ok. To build a new API to the following:
a. login with your facebook account
b. go to https://developers.facebook.com/apps
c. top right corner press Create new APP
d. you will be asked for the name of the app, link of website, description, etc.
4. In the bottom part you will have to select how this API will be used. Section is called: Select how your app integrates with Facebook. Here select Website
5. After API creation you will be provided with the API id, API secret (needed for Facebook Login).
6. Take all this information and insert in plugin settings. After that configure the rest of the FB utils (like button, like box, etc.)
The plugins uses OG tags, and has many options for ex to use custom tags for each page, or use global tags.
Very IMPORTANT: After doing changes to tags, you can test the way facebook shows the wall post from your website. You can load: https://developers.facebook.com/tools/debug
Keep in mind this: Facebook caches information recieved from your website. You need to clear the cache by loading the debugger above. If not after you make changes to OG tags, the wall post will look the same. This is maybe why none got any visible result after adding editing OG tags in theme header.php
For me it worked great. Now I can select the images i want to display from the page/post when making a wall post.
Waiting for your feedback.