cyoa_helper.user.js

// ==UserScript==
// @name        CYOA helper
// @match       https://*.*/*.png
// @grant       none
// @version     1.0
// @author      -
// @description Shift+Click to add checkboxes over images
// ==/UserScript==

function inject(){
  window.addEventListener('mousedown', (event) => {
    console.log(event)
    if (event.shiftKey) {
      const checkbox = document.createElement('input')
      checkbox.type = 'checkbox'
      checkbox.checked = true

      checkbox.style.position = "absolute"
      checkbox.style.top = `${event.pageY-6}px`
      checkbox.style.left = `${event.pageX-6}px`

      const size = 32;
      checkbox.style.width = `${size}px`
      checkbox.style.height = `${size}px`

      document.body.appendChild(checkbox)
      event.preventDefault()
    }
  })
}

inject();
window.addEventListener('DOMContentLoaded', (event) => {
  inject();
});