"Use the keyboard keys" could explain which ones

Maybe the text "Use the keyboard keys to flip between pictures in fullscreen mode." is not so helpful since it just say to use keys but not which ones.

You can browse through the pictures in fullscreen mode by clicking the <gui>Fullscreen</gui> button in the header bar or by pressing <key>F11</key>. Use the keyboard keys to flip between pictures in fullscreen mode.

https://gitlab.gnome.org/GNOME/eog/-/blob/master/help/C/view.page#L40-43