Fullwidth fixed background image for blog

Hello,

I have a blog with your theme “bloggera”.

I would like to have a fixed fullwidth background image. Once I scroll down to have a look at all post grid, I would like to still see the background image.

It would be something like on the picture I attach.

I can see that I can have a background image but it is related to boxed style, which is not exactly what I am looking for.

Is there any way? any plugin?

Many thanks for your help!