Sergiy Dybskiy provides a detailed tutorial on building a custom, accessible image lightbox within a GatsbyJS application, focusing on image optimization and accessibility. The guide explains how to use Gatsby's gatsby-image plugin for efficient image handling with a blur-up approach and GraphQL queries to manage image sizes and formats. The tutorial includes configuring the gatsby-config.js file to enable image queries, creating a Cars component to render images, and using styled-components for styling. It emphasizes accessibility by integrating the Dialog from Reach UI, allowing users to trigger the lightbox through image clicks. The tutorial is aimed at enhancing user experience in applications like e-commerce stores or picture galleries, encouraging developers to implement this feature and engage with the author for feedback or questions.