We are using the new image viewer. It has more navigation options, optional full screen view, optional carousel, and is part of updating Discourse to reduce duplication in the software it depends on.
If you have problems viewing images, please respond to this post with a link to the page with the problems, describe the problems, and say what operating system and browser you are using.
There are also some security fixes in this update, and some performance updates for certain views of lists of posts.