Insert background on Cheat Sheet Project

Hi everyone!

I need a little help on my Cheat sheet project.
I did some research on this project and on my question before but did not work out.
I like to add a new background to this project. I have found a really cool one and I have already tried all hints and tips on Google but could not figure out the solution.

This is the project what Im working on.

https://www.codecademy.com/practice/projects/independent-project-html-documentation

on the right hand-side I like to insert that blue thing as a background. It appears because I could paste the img link to the html sheet; but on the css sheet i tried all the tricks to appear as a right size with some light opacity behind the text.
I also have even tried to copy and paste the background from another completed project but that one either not worked.

22%20pm

Thank you for your help and reply!