Your distro will give you a number of choices for your desktop wallpaper, and you can get many more using from your . Here are some of the default wallpapers in recent versions (you can see the full-size images by clicking on the thumbnails):
Default in 4.5
or you can add your own pictures
The Cube
Blur
Cover Switch
Another Cube
Desktop Grid
Fall Apart
Flip Switch
Magic Lamp
Present Windows
KDE UserBase Wiki 
• 
• 