Eye of Gnome (eog) Review

You can set the current image as your desktop wallpaper directly from within this program

Pressing the ESC key exits Fullscreen mode, as well as exits the program ... hence press the ESC key twice and you're out

By default, double-clicking on the current image opens it in Fullscreen mode. This can be turned off in Preferences if desired

You can view a slideshow of all images in the current folder, and adjust the delay between images in the Preferences dialog