There is a command
tumme, that lets you view thumbnails of all images in a dir. Try it.
In emacs 24.x,
tumme is renamed to
Or, you can just call
image-dired, which will prompt for a dir first.
The generated thumbnails are stored at
Note: this may not work in Windows or Mac.
It's better to use a real image viewer or view them in desktop. See: Emacs Lisp: Open File in External App. It lets you press one key to open the image in a viewer or desktop.
thanks to xfq for his comment at Emacs HTML Tips#comment-269251723