While this isn't exactly what you want, try getting "Gallery" off of the Aminet. Point it to the directory on your local machine with the pictures, it creates the thumbs and a gallery page for you. It also creates frames, so navigation on the site is easy. It's straight html, so Amiga browsers aren't going to have any problem at all.
In fact, you could probably take the resulting pages, run them through an editor with find/replace, and add target="_NewWindow" to all of the picture links to get the pop-up effect.
Hope that helps.
