Problem is, it doesn’t say where to configure them – in Krusader Settings, I can’t find anything – and if they meant to configure some system file, I found I have a file ~/.config/mimeapps.lits which already contains items like
image/png=org.gnome.gThumb.desktop;xnviewmp.desktop;
image/jpeg=org.gnome.gThumb.desktop;xnviewmp.desktop;
Probably Krusader just doesn’t work so seamlessly with Gnome – I tried Live USB with Manajro KDE and the Krusader’s image viewer works there as expected.
It’s probably not worth the trouble, the gThumb seems to start fast enough and looks very nice. I was just used to pressing F3 when quickly viewing images.