How do I change the homescreen or launch background image?

To change the background on the home screen, navigate to Design - Smartphone -> Graphics:

Scroll down to the bottom of the page, and you'll see the Home Image / Launch Image. For this example, we'll use the Home Image option:

If you've opted for a template when creating the app, you may see existing graphics here, in this case, you can simply upload over the old image, or use the "X" option in the upper right corner to remove the existing image:

To add your image from this point, just simply c lick on the image itself to find and upload an existing file.