Changeable fixed image behind the text

[Certis]

Greetings.

 

I require assistance with something. I'm not entirely sure with what it could be solved, but I had a feeling that it would probably be JavaScript.

 

I'd like to place an image to some of my WordPress posts, that would appear behind the text, yet stay fixed there, like a background image.

 

By default, it should stay hidden, but when clicking a link (which is where JavaScript comes into play, I assume) it would appear in my post, behind the text, fixed, not up or down when people scroll.

 

I know this is a really selfish request for help, but I'd also like if I could somehow click on other links as well, further along in my WordPress posts, that would change that image and disable (hide) it, when necessary.

 

I would be extremely grateful if someone could tell me how to do that.