When scheduling posts, you'll see various options for including media, depending on which social network you're sharing to. In all cases, assuming you've included a link in your post, you'll have the option to choose an image from the suggested media that have been scraped from the link you're sharing.
We check if the image has a width or height attribute to determine if it's an appropriate image to include. If one of these is present, we will fetch the image and make sure it's both big enough and square enough. These images will be presented within the suggested media.
Image definition should look something like this:
<img src="link-to-your-image" width="600"/>
We do limit the number of images that are scraped from the link you're sharing, 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 this:
<meta property="og:image" content="link-to-your-image"/>
This is the recommended method for specifying a default image for viewers to use with your page.
Please note, these guidelines are for users who are sharing links to pages that they own and have control over. If you are sharing links to websites that you do not own, it is not possible to control which images are pulled in.