What backgrounds can be changed?
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1. All Storefront Pages (same background for all)

2. Storefront 'Content Section' (same for all)

3. Storefront Login Pages (where you can have multiple images)

To change all of these, go to the particular Storefront in the Admin System and go to the Theme. So:

Admin>Storefront Settings>Appearance>Theme>Edit Theme

Once in the Theme you can left click on any section and see the options for it on the left.

You can see the 3 background options above, on the left.

You can also get to this by clicking on the drop down:

For the Login Page backgrounds, scroll down to the bottom of the preview:

until you see the login page preview

Add an image, and remove all the others, if you like, on the resulting page: