Geeqie, one of my favorite image viewers for Linux:
Some may recall an image viewer from several years back, called GQview; Geeqie is a fork of that app. You can still find GQview in Arch AUR, but I don't think it's being maintained.
Running Geeqie, you can toggle back and forth to/from fullscreen mode by tapping the "f" key. However, I noticed that I couldn't get fullscreen mode to work in either LXDE or Openbox in Arch. I couldn't understand this, because it worked fine in Xfce on the same Arch installation.
When trying to go to fullscreen mode in LXDE or Openbox, I did get a second Geeqie icon on the panel, but that was it:
I started up Geeqie from the command line and tapped the "f" key to see what output I got:
From The Geeqie User Manual, section 10.3 ("Window Options"):
10.3.3. Full Screen
Selects the location and position of the full screen window. 'Determined by window manager' will leave the window placement up to the window manager. 'Active screen' places the window on the same screen as the Geeqie main window. 'Active monitor' does the same, but limits the full screen window size to the monitor containing the main window [...]
So, I went into Geeqie's preferences, to the Windows tab; in the "Full screen" section, I changed "Location" from "Determined by Window Manager" to "Active screen":
Now, Geeqie's fullscreen mode works fine in Openbox. Command line output now looks like this:
fullscreen.c:301: Original vindow is not visible, enabling std. fullscreen mode
Problem solved. Hope this helps somebody.