Poster Stack has an inbuilt and easy to use lightbox. This lightbox can be enabled or disabled per Poster Item. It will take all images inside one item and, by clicking on it, display the image in a lightbox.
To enable, select the "Enable Lightbox" checkbox on the Poster Item stack settings. Same for Markdown Folder Integration.
In addition, select "Show Captions" to display the image alt tag as caption.
You are able to define the alt text by double clicking on the image. For the Markdown Folder integration, use markdown syntaxt to add an alt text to your image.