I like the cover of "War of the outer gods", anyone know where i can get a larger version of it? Or of any FFG art (as wallpaper for PC)?

Generally, if you look up the illustrator you can usually find high resolution versions of the art in the artist's portfolio. Quite a few of them can be found on ArtStation if they don't have their own website, e.g. the cover of the Rules Reference is here: https://www.artstation.com/artwork/o8b84 and the cover of the Dunwich Legacy is here: https://www.artstation.com/artwork/LlqW0 .

Some artists also sell prints of their artwork if you want a physical copy.

Unfortunately, since War of the Outer Gods isn't out yet I don't know who the cover illustrator is and a quick search didn't turn anything up.