From Maties Ubuntu
Make your Ubuntu your own, with a unique look and feel.
- Move the image to the "Pictures" folder.
- Right-click on an open area of your desktop and select: Change Desktop Background or type wallpaper in the Dash.
- Select the location of the image by clicking on: Pictures Folder.
- Click on the image you want to be your wallpaper.
- Right-click on an open area of your desktop.
- Then select: Change Desktop Background
- At the bottom of the dialog, select Theme and modify accordingly.
- Ubuntu help wiki
- Advanced Customisation
- Theme websites