Experimenting with this and found that simply adding the .json file a config directory in the module seems to work, I assume the same might be true for themes.
Sounds similar to what I did on my website, though I did add padding and margins. In my case, I used Nivo slider placed at the top in the layout.
I'm no expert with Backdrop or css, but...
I am using Custom Blocks. I didn't know there was an Image Blocks module so after your suggestion I tried it but couldn't figure it out.
This will just be at the top of the header, not...
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Experimenting with this and found that simply adding the .json file a config directory in the module seems to work, I assume the same might be true for themes.