Include Blocks w/ Defaults in a Theme

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HI,
I am very new to C5. Sorry if this is asked and answered, I was unable to find it by search.

I want to build a new theme and include an image (or background image) that the user can swap out.

I see how to add an image block to a page after creating the page with a theme, but I do not see how to have the theme include an image block with the theme's default image.

Any help pointing me at helpful documentation, videos or samples will be greatly appreciated.

Thanks in advance,
Sean