After you click on the cogs icon you will see a popup window appear (as shown below).
Click on the Background tab
Click on the drop down field Background Style, and choose the value Image.
You will now see a image selector tool appear, this tool will let you choose an image file you want to set as your background image.
Click on the add image icon (box with a plus inside) below the label Background Image label.
Select an image from your computer.
Click on the Save button to commit your changes.
Add a background Image to a column
Setting the background image in a column is exactly the same process as described above for a row. However, you need to select the cogs icon from the column tool bar as shown below.
Click on the Background tab, and then follow the same procedure as described above for setting the background image of a row.