Set background-image depending on device

Dear all,
the requirement is to show a background image depending on the device.

I tried it this way:

Step 1 setting the jquery snippet into the survey flow <script>jQuery("#SurveyEngineBody").css({"background":"url('https://xxxxxxx.ca1.qualtrics.com/ControlPanel/Graphic.php?IM=IM_a3p7BQd8ya3hWx7')"</script>
step 2 calling the variable in the header section

Unfortunately, it doesn't work this way.

It would be great if anyone could help

Thanks for your support and best regards

Rudi