If you like uniformity in your Desktop and you are a KDE4 user most probably you'll be annoyed by the different wallpaper that is used by the KDM, the KSplash screen and your Desktop. In this guide I will show you how to set the same wallpaper for all three of them.
Set Desktop Wallpaper
First the Desktop which is the easier part. Just right click anywhere on the desktop and select Desktop Settings. Here just select your preferred wallpaper and click Apply.
Set KDM Wallpaper
Open System Settings > System Administration > Login Screen. In the Theme tab check which theme you are using. For me it's Horos. Therefore its wallpaper will be located in /usr/share/wallpapers/Horos/contents/images. There are different wallpapers in that folder, each one for a different screen resolution. Replace the default wallpaper with yours. Keep in mind that you have to keep the same name, format and resolution. Moreover you have to be root to paste your wallpaper in that folder.
Set KSplash Wallpaper
To set the wallpaper for KSplash I will use KSplashThemeGenerator which generates a splash screen for KDE4 from your current wallpaper, icon theme and plasma theme. If you can't find it your distribution's repositories you can download it from KSplash Theme Generator For Archlinux it is available in AUR
To install it manually after downloading it extract it and open a terminal where you have extracted its contents. Type:
Next select MyKsplashTheme from System Settings > Workspace Appearance and Behavior > Workspace Appearance > Splash Screen. If the wallpaper isn't the one you expected you can set it manually through ~/.kde4/share/apps/ksplash/Themes/MyKsplashTheme/ Once again you must keep the resolution and image file type of the original wallpaper.
I know that the whole procedure isn't very simple and fast but it's the only way I've found to set the same wallpaper and achieve a nice transition from KDM to KSplash to Desktop.