It is with sheepish bravado that I even communicate this.
I quickly created a more cat-friendly bg.jpg (sorry pups!) and followed the path to upload it in File Manager to public_html/assets/css/images. Although I could see it was there, those dang mountains wouldn’t disappear! I found another bg.jpg on public/html and deleted that. [This kinda refers back to original assist with public/html/img, but one level up. How many places should this bg.jpg sit!?]
Voila, deleting mtn jpg from the top of the hierarchy, while loading my cats under the path to which Alan pointed worked worked. Now I have my cats that still need a cleaning.
[Maybe this additional delete was an assumption - I am allowing you a glimpse into the neophyte’s mind.]
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Grab your Mountain Dew, cat lady, you are on the righteous path to sysadmin 
LOL (For real!) Just sharing because assume other Extenders (folks who all were intro’d at the same time) will have the same set up and perhaps need the same, extremely detailed level of instruction to change their mountains to molehills or whatever.
It’s awesome, with this stuff there is no other way to lead by example. And more times than not it just means be willing to try, fail, and fix. Hopefully this forum can help ease the turn around time from fail to fix.
Just because I like Peggy that much (even if she rejected my puppies), I updated the Ontario Extend Guide for Site Publisher, to have separate instructions for modifying the Personal, Aerial, and Under Construction themes
https://extend-domains.ecampusontario.ca/guide/calling-card/
As mentioned way at the bottom, beyond changing an icon or a background image, if you are wanting to go any farther it is time to consider banging on some HTML with a static theme., or trying a Wordpress site with a landing page.