I have a blog based on the Gatsby Starter Blog without any modifications.

The Problem

I want my posts to have a nice image when sharing them on social media.

A simple inspection of the generated HTML reveals that the Gatsby Blog renders (amongst others) the following meta tags:

<meta property="description" ... />
<meta property="og:title" ... />
<meta property="og:description" ... />
<meta property="og:type " ... />
<meta property="twitter:title" ... />
<meta property="twitter:description" ... />

It doesn't render og:image, image nor twitter:image, which are needed to properly share the articles on Twitter, Facebook, and other websites using Open Graph.

To see what our blog post link will look like on social media we can use Twitter Card Validator, Facebook Sharing Debugger, or we can simply read the Open Graph metadata using Open Graph Check.

The Requirement

We want to have a default (or fallback image) for the blog's home page and have the option to specify a custom image (even if it's not in the article's body).

The Solution

First let's add an ogimage property in the top section (frontmatter) of one of the blog posts.

---
 title: Hello!
 date: '2018-12-01'
+ogimage: './opengraph-default.png'
---

After restarting gatsby deveop, we now can add ogimage to the GraphQL query in src/templates/blog-post.js.

 markdownRemark(fields: { slug: { eq: $slug } }) {
   id
   excerpt(pruneLength: 160)
   html
   frontmatter {
     title
     date(formatString: "MMMM DD, YYYY")
+    ogimage { 
+      childImageSharp {
+        fixed {
+          src
+        }
+      }
+    }
   }
 }

The next step is obtaining a file path for the ogimage property in a blog post, and then passing it to the SEO component. Still in src/templates/blog-post.js:

 class BlogPostTemplate extends React.Component {
   render() {
     const post = this.props.data.markdownRemark
     const siteTitle = this.props.data.site.siteMetadata.title
     const { previous, next } = this.props.pageContext
+    const { ogimage } = post.frontmatter
+    const ogImagePath = ogimage && ogimage.childImageSharp.fixed.src

    return (
      <Layout location={this.props.location} title={siteTitle}>
        <SEO
          title={post.frontmatter.title}
          description={post.excerpt}
+          image={ogImagePath}
        />

The SEO component (in src/components/seo.js) now needs to accept an image prop.

- function SEO({ description, lang, meta, keywords, title }) {
+ function SEO({ description, lang, meta, keywords, title, image }) {  

If the image prop is undefined SEO will use a default value, which we need to import as a static image:

+ import defaultOpenGraphImage from '../../content/assets/opengraph-default.png'

...and then we use that as a fallback value.

 <StaticQuery
   query={detailsQuery}
   render={data => {
     const metaDescription =
       description || data.site.siteMetadata.description
+    const ogImageUrl = 
+      data.site.siteMetadata.siteUrl +
+      ( image || defaultOpenGraphImage )
     return (
       <Helmet

The data.site.siteMetadata.siteUrl expression above is needed to prefix the file path with the website's URL, but it will be undefined until we add siteUrl to the GraphQL query at the bottom of seo.js:

const detailsQuery = graphql`
  query DefaultSEOQuery {
    site {
      siteMetadata {
         title
         description
         author
+        siteUrl
      }
    }
  }
`

Now we can use ogImageUrl in the metadata:

  {
    property: `og:title`,
    content: title,
  },
  {
    property: `og:description`,
    content: metaDescription,
  },
+ {
+   property: `og:image`,
+   content: ogImageUrl,
+ },
+ {
+   property: `twitter:image`,
+   content: ogImageUrl,
+ },
+ {
+   property: `image`,
+   content: ogImageUrl,
+ },

And that's it!

We added a property to the frontmatter of blog posts. We then modified the GraphQL query in blog-post.js to read the property and passed it to the SEO component in seo.js. The SEO uses an imported image as fallback and then it renders the necessary meta tags using the post's image or the fallback one.

You can see a Pull Request with all the changes I did for my blog: https://github.com/JulianG/juliang-blog/pull/3/files