Zakra theme allows to change the Background Image within both inside and outside the container. Just follow these steps:

Go to the Appearance > Customize > Global > Background Image section from your WordPress Dashboard.
Now click on Select image and choose the required Background Image for your site.
Click on Publish.

Note: The effect of this change is visible only in the Boxed Container Layout. To learn how to change the Container Layouts, you can consult this article: How to change the Container Style?

It is shown in the image below:

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