Facebook no longer allows you to change the image associated with a link post on your page. This is to reduce Fake news and manipulation of headlines.
You can get around this.
If you really want to.
1. Create an ad in Facebook Ad Manager. – In the ad you can choose the link and make changes to the image, headline and copy.
2. Start the ad and immediately go back into ad manager and pause the campaign – if you want to do this often, just put the ads in an already paused campaign so you don’t spend any money on it.
3. In ads manager go to the “Page Posts” tab.
4. On the left hand side choose “ad posts”
5. Check the post you just created.
6. Click the “Actions” button and choose “publish”
7. The post will publish to your page timeline.