I have various problems with Desktop Widgets and Screensaver lockers in current Debian testing (KDE 4.10.5). Only the simple locker works stably. Is there a way to customize it by changing the wallpaper?
Thanks.

not sure how it works in debian but on kubuntu you go to /usr/share/wallpapers/Elarun/contents/images/ (elarun being the name of the default wallpaper) and replace the 2560x1600.png with the image you want

Thanks, this makes sense. But I guess to read the default wallpaper itself is hardcoded in the simple locker?

i think its hardcoded to display the default wallpaper yes, not sure...replacing the default wallpaper with the picture of my choice worked here at least..

The default wallpaper is determined by the current Plasma theme I believe.

Confirming that the wallpaper for the simple screen locker is determined by the plasma theme.
For example, I found a theme called Mirado which specifies a different wallpaper.

Upon examination, the theme has a [Wallpaper] section in its main metadata.desktop file with keys such as defaultWallpaperTheme, defaultFileSuffix, etc. The Mirado theme provides its own wallpaper theme, but any wallpaper theme could be used, you just have to add the [Wallpaper] section to the metadata.desktop file of your current desktop theme. Of note, a "wallpaper theme" may not be just a single wallpaper (which is a bummer), but it has to be one of those folders which contains the certain metadata about the wallpaper and the collection of dimension-specific copies of the wallpaper image.

It would be nice to be able to have a UI for configuring the simple locker wallpaper separately (preferably in the screen locker dialog, with such options as: use desktop theme wallpaper, use default wallpaper, use current wallpaper, pick wallpapaer)

Feature requests should be made at bugs.kde.org, or discussed in the Brainstorm on this forum.

Added my suggestions to the existing bug (feature request): https://bugs.kde.org/show_bug.cgi?id=312828