I use Gwenview as my base or default viewer for images . I find it is reliable, and with very good prformance, and includeds necessary basic features such as re-size, copy, move.

Gwenview is an excellent browser and viewer

and it goes beyond that in that it also features OPEN WITH which allows you to open an image in RawTherapee, or GIMP, or KolorPaint -- which in effect creates a "studio" -- all open-source and at n/c

Gwenview also has an excellent "share with" interface if you like posting to Flickr, etc (it has a bunch of interfaces )