1. Login to the backend of your site.
2. Click on Posts from the sidebar navigation. Either create a new post via the blue button or edit an existing post depending on your preferences.
3. Add in all of your content, title, etc. to the post.
4. Scroll down to the Yoast SEO section. Click on the social tab on the left side of the section and it will pull up the tabs for Facebook and Twitter.
5. The last field is for your image. Click on the button that says Upload Image. Chose the image you want from the Media Library or add one in if it’s not already in there. Make sure the image you choose is less than 8MB.
6. Once an image is chosen and the post has all of the needed information, click the blue Publish or Update button in the upper right hand corner.