can somebody please show me how to make images clickable thumbnails of uniform size for an image marquee?

You can use a server side language to resize the images to a defined thumbnail size. I work with PHP mostly and this is done easily with the GD2 Library, or Imagemagik.

Once you have the thumbnails you can place them in a marquee and have an onclick event handler attached to the thumb or anchor tags (<a></a>) wrapping the thumb. This onclick even would then open the thumbnails.

There is a great PHP/JS based implementation of this called lightbox.
http://www.huddletogether.com/projects/lightbox/