Gwenview calls itself an image viewer but it is actually a lot more than that as it can crop, rotate, mirror, flip, and resize images as well. It also supports basic file management action such as copy and paste. What really makes it stand out though is that it is fast.
Gwenview comes preinstalled in Kubuntu and can be found under Menu/Applications/Graphics or by searching in the menu.
If, for some reason, it is not installed it can be reinstalled by opening Konsole and typing,
sudo apt install gwenview