feh is an open-source image viewer that is most aimed at command-line users. It can be used to view images on disk or, more often, for setting the desktop background for tiling window managers like i3. It has several command line options to change the behavior for either of these uses, mostly entered around how to resize the image for the view.
USE flags for media-gfx/feh A fast, lightweight imageviewer using imlib2
emerge --ask media-gfx/feh
To use it to set one's desktop background to a scaled version of a wallpaper:
feh --bg-scale /path/to/wallpaper
where /path/to/wallpaper is the path to the wallpaper one wishes to use as desktop background. Likewise to set the desktop background to a tiled version of the wallpaper:
feh --bg-tile /path/to/wallpaper
Adding these commands to ~/.xsession is an easy way to have the wallpaper set when X is started. Alternatively cron can be used to change the background in a time interval:
*/5 * * * * DISPLAY=":0.0" feh --randomize --bg-fill /path/to/wallpaper_library
This will run feh every 5 minutes, and every time randomly chooses a picture from the directory /path/to/wallpaper_library. The environment variable DISPLAY must be set inside of cron because cron was started before X. For more help with automated tasks on intervals, see the page for cron.
feh can also be used as a standalone image viewer. feh can accept a single image, a number of images, or a directory as input.
|Left Mouse Drag||Pans the Image|
|Right Mouse Click||Secondary Menu|
|Scroll Up||Previous Picture|
|Scroll Down||Next Picture|
For further details on its usage see The ArchWiki's article on feh.