To change the image that shows up in the “card” when you share a link to your post, you can hide an image in your post, say by using the Post Signature feature. You can add something like the html tag below:
<img src="yourImage.jpg" width="250" height="250" style="display: none;">
What that does is it will change the content for the og:image meta property on your post. At the same time, since the style is set to display: none;, the image won’t show up on your post. This is useful for those posts that don’t have an image in the first place, but you want a specific image to show up when sharing links to your site.