A Simple and Fast Image Viewer for X. Part of the LXDE project.
Ranked in these QuestionsQuestion Ranking
Pro Multiple windows
When opening multiple images at once it opens every single picture in a separate window.
It has all the necessary functions that you can reasonably expect from a lightweight image viewer: you can view all the image files in a directory, rotate and save them.
Pro Extremely lightweight
And still you can browse all images in a directory.