This is a quick edit of the offical Gatsby blog starter, turning the field named "images" into a special one that takes an array of filenames and returns their respective ImageSharp nodes.
It's a little slapdash, but it serves its function as a proof-of-concept.
Implementing this into your own site may take some alteration- check out the createSchemaCustomization method of gatsby-node to see how I implemented it here.
I added some example images to the "Hello World" post. Fire up the development server and check it out to see it in action!