RMB click on destop and select change desktop background. Open /usr/share/backgrounds as admin, and put in any background you want. Or RMB click on any image you want to set as background. You can get desktopnova through apt and have you back ground change every 10 minutes as well.
The OS: Linux Mint 17.3 KDE
The Hardware: Dell 17R Special Edition, Intel Quad Core, i7 - 3.40 GHz, 8 GB Ram, HD 1TB
Video NVIDIA GeForce GT 650M 2GB
The Name: Bob