When comparing Eye of Gnome (eog) vs Phototonic, the Slant community recommends Eye of Gnome (eog) for most people. In the question“What are the best FOSS image viewers for UNIX-like systems?” Eye of Gnome (eog) is ranked 9th while Phototonic is ranked 15th.
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Pros
Pro More lightweight than other kde or gnome dependent applications like gwenview, gthumb or kphotoalbum
Pro Double-click to view Fullscreen
By default, double-clicking on the current image opens it in Fullscreen mode. This can be turned off in Preferences if desired
Pro Image Slideshow
You can view a slideshow of all images in the current folder, and adjust the delay between images in the Preferences dialog
Pro ESC key exits Fullscreen and program
Pressing the ESC key exits Fullscreen mode, as well as exits the program ... hence press the ESC key twice and you're out
Pro Can set image as wallpaper
You can set the current image as your desktop wallpaper directly from within this program
Pro Clean and functional UI
Beside being pretty clean and easy-to-use, the UI is also pretty fast.
Pro Can be easily keyboard driven
Lots of pre-defined keyboard shortcuts.
Pro Highly customizable
Lots of options to personalize your experience. You can set Photonic to fit images by width and height, or width or height; rotate according to Exif orientation; show/hide image name in viewer; adjust space between thumbnails; adjust delay between slides.
Cons
Con No APNG support
And it won't be it seems https://bugzilla.gnome.org/show_bug.cgi?id=151281
Con Still have gnome dependencies
Con No Dark mode
And not a lot of customization available.