That's pretty close! That bottom part of the tutorial does not tell you how to call one for desktop and one for mobile. It would show the same image in both.
Here's the really good news Pablo! You've been on here enough to put a link here, yay! I'm really glad you want to do it yourself too as that is way more satisfying than having someone do it for you. Please put a link to your blog or even make a tester blog like I did above so we can see what you're working with. Please post links to both images too! That would help a lot.
If just basic how to call the different layouts would help, you would use conditional tags.
<b:if cond='data:blog.isMobile == "false"'>
what you want for desktop code would be here
what you want for mobile code would be here