Hello @eursula.
There are many ways how to approach this and it is not very much specific to Sage I would say.
For example:
-
use featured image for Frontpage if your front page is set in Settings -> Reading -> Front Page as a static page. Then across all pages just check for featured image and set it in body tag. Of course put your code in App controller or hepers.php depending on what you wanna do exactly.
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Same would work with ACF, with more flexibility here if you perhaps need different / multiple images etc., and you can pass all ACF data to views via
protected $acf;
inside the class. See ACF variables in blade template too. -
Depending on if the background image is different for each page: if it’s the same, you could set it in your custom “options” page which you can create easily with ACF Pro as well: https://www.advancedcustomfields.com/add-ons/options-page/.
Do you need more specific info? What is your exact problem?