I checked and it seems like Lichess definitely made it inbuilt. This comes as a big surprise to me because many years ago when I created my account, I set that specific image manually as the background image. It wasn't the default image back then as far as I remember. As you can see, you can change the image by providing a link to it. I have to look at the source code to verify that, though. Fortunately, Lichess is open source.

On the other hand, I'm not accusing you, LOL, but I understand that's what it might seem, because of my ignorance that it's a Lichess feature nowadays (in any case the image isn't even mine). So, to clarify: since the image is a Lichess feature, it's up to you whether to use it or not. I'm just suggesting you to have your own original style overall along with your good content.