Version: (using KDE 4.0.4) Installed from: Fedora RPMs in System settings -> display it looks like it is not possible to remove the background image and have a solid (or gradient) colour only. this was possible in kde 3.5.4 but seems to be impossible now.
> System settings -> display ?
From the K-meny (equivalent to start in windows). press the K-meny button, then press Settings, then System settings and then Display.
or you can simply right click on the background of the desktop and select "Configure desktop".
Right click on desktop -> Desktop settings -> Select "None" in the dropdown under Wallpaper, then pick a colour.
there is no None option. that's the problem.
There definitely is in trunk. So this will be fixed in 4.1 if it isn't already.