Lots of image managers are either resource heavy, or not suitable for viewing whole folders comfortably. If you're looking for something in between a viewer and a full blown image manager, try Sequential.
It is fairly basic, but it's been specifically designed with viewing in mind. It looks great, and most importantly allows you to flick through images quickly and easily with no noticeable loading. There are no library management or editing features, but it works great for viewing folders of images in order (hence the name Sequential).
Sequential is compatible with most image formats, and compressed folders too, including the comic book formats CBR and CBZ. While it's good for viewing comics, there seems to be no way of viewing images side by side, meaning you can't read a comic with a double spread. That's pretty rare though.
Sequential is a slick image viewer, that's ideal for showing off your holiday snaps or reading a comic due to its lightweight and responsive nature.