click on add a widget
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select "IMAGE".

Then go to "Upload picture" and click on the desired picture.

 On the top right you can see the "dimension" of the graphic, write it down (do not copy it) for a later step.

With a right click on the desired image you go to "examine".

Copy the link at "background-image" by right-clicking with the cursor on the blue link and go to "Copy address".

Close everything until you are back in "Edit Design" and go to My CSS Styles, top right. 

Go to "tasks" - "edit"

Choose "CSS" int the menu

Scroll down a bit until you get to "background URL" and replace the old URL with the new one.

IMPORTANT! Now enter the "dimension" which you have noted before. NO Copy & Paste!
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Save & Close.
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Last but not least, delete the widget you added at the beginning!
The widget is usually located at the bottom of the "Edit Design" menu.