Hello,
If you want to change the background image of the promo pop up then you can add from Theme Settings >> Shop >> Promo popup.
Screenshot for Clarification: https://ibb.co/9nspNDM
If you want to edit the content inside the promo popup then you need to edit the HTML block used in the promo pop text.
OR
If you want to use a header banner you can add image as background from Theme Settings >> Shop >> Header Banner.
Screenshot for Clarification: https://ibb.co/dbKn6Wg
If you want to ask anything else then provide with some relevant screenshots to check it myself and help you out accordingly.
Best Regards.
.
