Hellbox is a lightbox coded in 666 bytes, read the HellCode manifesto. Keeping it simple, use this 666 bytes long vanilla javascript like
<a href="big_image.jpg" class="hellbox" target="_blank">
<img src="thumbnail.jpg">
</a>
Clicking the thumbnail.jpg shows big_image.jpg in a lightbox. Clicking anywhere closes it and returns the user to the page. If the javascript is forbidden, the big_image.jpg shows in a new tab as it should. Nobody expects any other feature, so don't add any. See also a live demo.
This product is distributed under the HELL LICENSE v6.66, please read attached LICENSE.md file.