qView simple and fast image viewer, I don't need the bells and whistles of most image viewer apps, qView is a simple to the point app like old feh but integrates well will KDE.
I use Strawberry too and in general it is great. Sadly it still lacks some features I need, so I still need foobar2000 with wine on the side (for library management, stuff like viewing and editing all custom tags, renaming files according to tags instead of copying them, ...)
Firealpaca is how I discovered kde and I don't see it mentioned too often, so I will take my chances and say that. However it's the only kde program I know of that isn't FOSS. But I typically prefer it's simplicity over the bells and whistles of gimp and sometimes krita.