You can upload a background image for your store by going to the "Storefront" tab and then choosing "Banner and Background Image".
If you choose to have a background image in your store, you can change the settings for how it displays from the "Storefront" tab in your Admin Panel. Choose the "Theme" section and then scroll down to the "Styles" section.
You can set a plain background color using the "Body Background" setting.
Right at the bottom there is a setting to hide the background image completely. If you have set a background color this will still show even if you hide the background image.
Background Image Repeat
- Repeat - This means your image will be tiled across the screen. No matter how big your image is, it will be repeated as often as is needed to completely cover the viewer's screen, no matter how big the screen.
- Repeat X - This means if the viewer's screen is wider than your image, the image will repeat itself across the screen.
- Repeat Y - This means as the viewer scrolls down through your store, when the bottom of your image is reached it will repeat again.
- None - This means your image will only be displayed once on the page. If it isn't big enough to fill the screen, your background color will show when the image has run out.
Background Image Position
If you don't have your background image repeating, this will determine where the image is placed on the viewer's screen. Bear in mind that different people use different sized screens, so unless your image is centered it will appear in a different place depending on the screen it's viewed on.
Scroll - This means that as the viewer scrolls down through your store, your background image will move up with the page.
Fixed - This means the background image will stay fixed in place, even if the viewer scrolls through your store. Although your store page will move up, the background image will not.
If you want to easily try out these different options and see how they affect your store, just use the design toolbar.