Why not a plugin? Two reasons. First, I don’t know how to code plugins for wordpress. Second, wordpress philosophy of external features doesn’t go beyond plugin category (and I think it is reasonable in many aspects) and I don’t think it is a good idea to make plugin for plugin.
UPDATE: Now Lazyest Slideshow function transformed into PlugIn – Lazyest Gallery Thumbs Slideshow
How it works? Simple. Function code goes through all corresponding thumbs folders in root of the gallery and takes 2 random pictures from each thumb folder and forms array of thumb images and corresponding links to real images.
Array is preloaded when visitor opens website. Thus, the more folders the bigger array and loading time, but, you can change the number of pictures for each folder as well as the number of folders to use. For example, five folders give 10pics, each 3-4kb – not much to feel uncomfortable. And, finally, all these thumbs are dynamically rotated in a slideshow with nice fading effect!
All images are linked and clicking on it opens Lazyest Gallery slideshow main window with that particular image.