jQuery is JavaScript framework, which makes very easy development of JavaScript applications. Moreover jQuery will guaranty you that your application will work on most popular web browsers used today.
Java Script is very popular scripting language tool, because he use small source code, cross platform within many browser and the execution of the script is fast for this, because calculation of the script is made on user's computer and your server will stay available for main applications.
If you have some photo gallery, with many folders of your events and each folder give to user a list with all photos you probably want to display one large photo and the others are small (thumbnails) in some list bar.