swayimg is just a really simple image-viewer. Previously MARBS used imv but due to outdated dependencies and sparse maintainance it got superseded by swayimg. It is controlled by the keyboard with the vim style keybindings.
Documentation
man swayimg - for general overview of passable cli options
man swayimgrc - for configuration file options
Running
swayimg is opened automatically when you select a photo file to open from lf or another program.
Obviously you can run it from the command line by running swayimg file.png, etc.
Configuration
~/.config/swayimg/config– custom key bindings.