The selector you need to target the element with the background image is this:
.fl-node-5c177bf341a41 > .fl-row-content-wrap
You can add selectors manually via the selectors menu (top left) or by customising the suggested selectors in targeting mode (with the advanced options expanded).
It’s worth watching the Targeting video tutorial which is accessible from the Help menu at the top right of the Microthemer interface. This shows how to move up to parent elements via the breadcrumbs, HTML code, or using the directional icons to the left of the selector label. Using these targeting adjustment options would be an alternative to manually adding the selector, as I suggested above.
If you have the Background properties on the Computed tab visible when move up to parent elements, the background-image will be displayed when you are on the correct element. The Computed tab will be present when the advanced targeting options are showing BTW.
I hope that helps. Please let me know if you need further assistance.