How to properly create Type definitions on Gatsby for external images

Question

I am using Gatsby 3.6.1, with Gatsby plugin image. I am fetching images from an API which returns the following structure:

[
   {
     "pk": 1,
     "url": "https://location_of_the_image1",
   },
       {
     "pk": 2,
     "url": "https://location_of_the_image2",
   },

]

These images are stored within the nodes API__Image and then I use createRemoteFileNode to create a local version of the file within the API__Image.image field:

exports.onCreateNode = async ({
  node,
  actions: { createNode },
  store,
  cache,
  createNodeId,
}) => {
  if (node.internal.type === `API__Image` && node.pk) {
    let fileNode = await createRemoteFileNode({
      url: node.url, // string that points to the URL of the image
      parentNodeId: node.id, // id of the parent node of the fileNode you are going to create
      createNode, // helper function in gatsby-node to generate the node
      createNodeId: id => `IMG${node.pk}`,
      cache, // Gatsby's cache
      store, // Gatsby's Redux store
    })
    // if the file was created, attach the new node to the parent node
    if (fileNode) {
      node.image___NODE = fileNode.id
    }
  }
}

The problem with this setup is that it returns the following error when using gatsby develop or gatsby build more than once:

Invariant Violation: Encountered an error trying to infer a GraphQL type for: "image___NODE". There is no corresponding node with the "id" field matching: "IMG286,IMG287...

How should I create a proper type definition for this code? I have tried following the example of this page, but I keep getting the same error:

exports.createSchemaCustomization = ({ actions }) => {
  const { createTypes } = actions
  const typeDefs = `
  type API__Image implements Node {
    pk: ID!
    url: String!
    image: File @link(from: "image___NODE")

  }
`
  createTypes(typeDefs)
}

Another interesting question, why the node internal type is called API__Image but nonetheless when accessing GraphQL at http://localhost:8000/___graphql it is displayed as apiImage? Should I use API__Image or apiImage when defining the GraphQL Type?