When looking for a script to make an Image Gallery. The best option I have found is Mike Wheeler's "Picture Frame".
It's a very simple, but hard minded, script. It's only one file.
Very easy to configure.
You only need to make the thumbnails by your own and name them the same as the original files. When you upload your files you must have different directories for images and for thumbnails (it's obvious).
One of the advantages of this script is the ability to grow up your image gallery.
You only have to make the thumbnail for the new image and upload both files to their respective directory.
The next great advantage is the easy way to customize the generated HTML.
You can (I make it this way) create a HTML document as you'd like your gallery to be shown. Then you can copy-paste the code in the script. It uses two variables : HEADER and FOOTER. There you copy the code that goes before and after the thumbnails table or the Image.
Otherway, you can leave the script as it is, and just configure the URL and PATHS needed for the script to look the files in.
If you like it enough, you could ask Mike for written permission to make some modifications.
Give it a try.