Using different images on a page - desktop vs mobile

Hey @tuanphan @Jia

I am having an issue and you guys have both helped me immensely in the past so I am hoping to get your direct advice.

I need to make one image show up in the first section on my homepage when I am on desktop, 

and then ANOTHER, different image show up in the same section, on the homepage, but this time - on mobile only.

I have tried a tutorial and done the following CCS (but in the page code heading injector): 

But it didn't work.

This is what I currently get:

I want to use this for mobile:

and this for desktop:

Hi,

What is site url?