1. System Settings -> Display and Monitor -> Screen Locker -> Screen locker type: Desktop Widgets -> Configure... -> Settings -> Wallpaper -> Type -> Image -> Open... 2. Can't type anything in any field in the file selection dialog. Reproducible: Always
All keyboard input is disabled (tabbing between widgets, etc.)
kcm_screensaver reached it's end of life with Plasma 4, whose last bugfix release was last August 2015, we are sorry we did not get to fix this bug. In Plasma 5 we support screen lockers instead of screen savers. Sorry again we could not fix your bug.