Sadly Trent no longer appears to be maintaining the code, so it is now becoming limited in its usefulness. In particular I have had requests recently for simpl_gallery to display PDFs, and to allow for image collections on external storage sites, which is not easily achieved with the current code.
So I made the decision to look for an alternative framework. It looks like “Galleria” from galleria.io is going to be the most powerful alternative. It can include images from Picasa web, and Flickr, as well as PDFs.
I currently have a demo gallery of what will probably be known as “simpl_gallery3” now running on the kiwitrees demo site . Please take a look and let me know here if you see any issues, or improvements. I aim to release it around the same time the next upgrade of webtrees happens (w1.4.2)