Suppose your page URL contains the query string like this in the page URL: ?slide=2 (or &slide=2) . Then you can add the following script to your page that will direct the image slider to show the specified slide when the page loads.
Note: The index number in the Ninja Slider built-in function, nslider.slide(index), is 0-based. So 2 means the third slide.
The above code block can be added either inside the <head>...</head> section, or within the <body></body> section.