In plasmashell I noticed very poor quality of displaying the wallpaper images. They seem like being very aggresively jpeg-compressed - blurry, lacking details, sometimes artifacted.
See the attached screenshots with the very same image shown as desktop wallpaper and gwenview at ~ the same scale.
The wallpaper looks ugly
The wallpaper looks like it should
Created attachment 93329 [details]
Created attachment 93330 [details]
The desktop image size exceeds the screen resolution and the positioning is 'Scaled & Cropped'. I never noticed such degradation of the image quality in plasma from KDE 4, not other DE's.
Positioning: Centered and Tiled don't work at all (see the additional screenshots)
Created attachment 93331 [details]
broken Tiled positioning
Created attachment 93332 [details]
broken Centered positioning
*** This bug has been marked as a duplicate of bug 338506 ***