WP ImageViewer

This plugin expects the same html layout as lightbox, colorbox and fancybox. The image/thumbnail should be inside a link, an a element. That link should point to the main media image, not the attachment page or a custom link. Only when the link points to an image file, it will show the viewer,

You can go to Gallery > Other Options > Lightbox Effect-tab and select None for the Lightbox Effect.

This code should help you, you can change these default values and put this PHP code in the functions.php file of your theme (preferably a child theme). imageviewer_custom_options = { zoomValue: 100, snapView: true, maxZoom: 500, refreshOnResize: true, zoomOnMouseWheel: true };