Adding a background image to your storefront is a great way to create complete branding and a cohesive look.
Go to the Design tab in the design panel and scroll to Backgrounds. Select the checkbox next to BG Image and click the blue link titled "Choose image."
Browse through the images on your computer and select the file you'd like to upload. Your image will be added to your image library and selected as the new background for your store.
After uploading your image you can customize the background's design with the following options in the design panel: BG Color, Image Tiling, Image Alignment, Image Scrolling, and Image Size
The maximum recommended dimensions for background images are 1680x1050 pixels.
You can also modify the background color of your store and pages. Click the BG Color option to add a color and opacity level to your background. This is a great feature for those that have intricate backgrounds which at times can interfere with legibility behind text.