follow the below steps
1.Load your pictures into Photoshop.
2.Click "Image" then "Image Size" to access the Image Size menu.
3.Reduce the size of the image by entering new height and width values.
4.Select "File" then "Save As" to save a copy of the image with a new file name.
5.Open the website that will display the thumbnails. You can do this using web page authoring software or by coding the HTML by hand in a word processor.
6.Insert the first thumbnail where you want it to go in the page.
7Convert the thumbnail into a link to the full-sized image, set to open in a new window.
8.Save the file and test it in a web browser.