Getting the right thumbnail for your post
If you are the site owner
The way we know that an image in your page is appropriate to use if it has a width or height attribute. If this is present we will fetch the image and make sure it's big enough and square enough. If all these are in order we will present the image to people linking to your site with Buffer. So, your image definition in your page should look something like this:
<img src= " the url for your image " width="150"/>
where the actual value for the width should correspond to the size of your image.
However, we do limit the number of images we will scrape from your site, so there is an additional way to ensure that a given image is presented as the first option regardless of size and aspect ratio. This method is to use Facebook's OpenGraph meta tags like so:
< meta property="og:image" content="https:// LINK-TO-YOUR-IMAGE"/>
This should be placed in the section of your page and is actually the recommended method of specifying a default image for viewers to use with your page. That should fix you right up. Set this for future images and I think you should be squared away!
If you are not the site owner
The images that are presented for your selection are based on the format of the web page you are viewing. If you feel that the images are incorrect or could be improved, the site owner will likely need to make some modifications to the page. Your best bet is to point them to this FAQ, and in particular the section above.