How to let some images at the very background??

Hiii!! I'm editing my blog, and I'm having this specific technical problem :p. I've seen a lot of users that have some images in the background, outside any box, and I can't help but struggle with them because I can't make them. Probably just a typing mistake; maybe I'm copy-pasteing something on the wrong part of the code. IDK heeeeelp TmT

Here are some examples in case my explanation turned out lame (probably it is considering that English is not my first language, lol) and TYSM in advance!!