Media queries for responsive design, and css variables.

I found it difficult to figure out how to display two images, or how to have both desktop and mobile images in the same html file.

I'm unsure about how I wrote my css in order to position things, feels like I can write much better css, but I learned a lot which was great.

Would like to know where or how to store or how to work with different image sizes when going from desktop to mobile.