Here's how it all works.

A program (index.php) looks through the folder 'slides' and makes a list of the pictures there, jpg and png. (therefore you can add and remove pictures at will and the page doesn't need updating)

It generates the document (the page you're viewing right now) based on the pictures it found, but instead of this document pointing at the pictures, it points at another program (thumb.php) which creates thumbnails on-the-fly. (it will also cache the generated thumbnails to conserve CPU resources, so it really only generates the thumbnails the first time a newly-added picture is requested, and delivers the pre-generated thumbnails if they exist) It breaks the list down by those that are taller than wide, and those not.

I use the jQuery javascript library on this page (and others) and jquery.fancybox.js for the full-size overlays, along with jquery.dropshadow.js for self-explanatory effect.

If you are interested, the whole (54k) package can be downloaded here. Just untar the file to your web hosting, everything is contained within a folder named 'slideshow'. Browsing to that folder will then display thumbnails of all PNG and JPG images in the sub-folder 'slides'. *Note that if your web server lacks php-gd, thumbnail creation will NOT work, so you must either have php-gd available OR populate the 'thumbs' subfolder inside 'slides' with manually-created thumbnails named 't_'+original name. (pic.png is t_pic.png)