So ...

Upon activation, go to Appearance => Fullscreen BG Image.

This is where you can upload your background image.

Simply click the "Choose Image" button, and upload the given image from your computer to your Wordpress media library:

It is recommended that you use an image that is at least 1600×1200 px.

Finally, don't forget to click the "Save Options" button ...

OK. Now let's see the result ...