Element page text jellyfish backdrop

Hi I am trying to set a background image to the text of a page, but I don’t know what to fill in at element to get it there. I have tried a few different things I found, but don’t know a lot about this to know exactly what my options are let alone which the answer is.

If there is anyone who can help, I would greatly appreciate it.

an example of such a page is:
https://thewhole.net/?page_id=183