Some of the landing page designs do allow you to change the background image. You can replace the background image of your landing page from the design view edit screen. Click the Change background image button on top of the landing page screen. On your left side under customize area, you should see some options to replace the background image.

Image URL field allows you to directly paste the URL of that image that you want to use as background of the landing page.

You can also upload a background image from your hard drive. Click the orange “Click here to upload image” button and select the file from your hard drive. Click Upload button and it will replace the background image on your landing page.