gwenview: KDE image viewer,
eog: Eye of Gnome, the Gnome default viewer,
xviewer: Eye of Gnome successor for MATE and Cinnamon,
ristretto: XFCE image viewer,
gpicview: LXDE image viewer,
lximage-qt: QXDE image viewer,
xdg-open: generic file handler
The problem with xdg-open is, that it calls the browser (firefox) as fallback. This is not good for DVI and PDF, but still better than an editor (Gimp) for raster images.
Kee Gimp as last option for viewing, and default choice for editing.