Callback signature:

array &$images

Arguments:

1. array &$images - the image array

Data structure:

[
  "string-addon-unique-identifier" => [
    [
      "info" => [
        "nm" => "raw-html-for-image-name",
        "by" => "raw-html-for-image-author",
        "lc" => "raw-html-for-image-license"
      ],
      "image" => [
        "thumb" => "url-to-image",
        "full"  => "url-to-image",
      ]
    ]
  ]
]

Data structure example:

[
  "my-addon" => /*start_your_copy-paste_here*/ [
    [
      "info" => [
        "nm" => "Dirty leaves",
        "by" => "Nicky",
        "lc" => "Creative Commons 4.0"
      ],
      "image" => [
        "thumb" => "styles/nicky/backgroundpack1/dirtyleaves_small.jpg",
        "full"  => "styles/nicky/backgroundpack1/dirtyleaves_full.jpg",
      ]
    ],
    [
      "info" => [
        "nm" => "Clean leaves",
        "by" => "nonCa",
        "lc" => "Proprietary"
      ],
      "image" => [
        "thumb" => "styles/nicky/backgroundpack1/cleanleaves_small.jpg",
        "full"  => "styles/nicky/backgroundpack1/cleanleaves_full.jpg",
      ]
    ]
  ] /*end_your_copy-paste_here*/
]
Tip: Try doing something near this as the last line of your event listener PHP code:
$images["my-addon"] = $myImagesArray;