feh is a text-based image viewer. This means it has no graphical user interface. You can open a terminal and type
feh --zoom 100 example.jpg. The software will zoom this image to 100%. You can also leave the filename empty. Feh will walk through the actual directory and show every image.
Gimp does not open a file at 100% zoom every time. But you can open an image and immediately press 1. This will zoom the image to 100%.
display from ImageMagick is also text-based. Use
display example.jpg to open an image with 100% zoom (programm does it by default).