images version for mobile ?

You can use CSS to tailor your content to specific device widths using media queries:

@media screen and ( max-width: 400px ) {
  #masthead {
      background-image: url(path/to/image.png);
  }
  /* The rest of your CSS for screens with a max width of 400 pixels goes here. */
}

There’s more in-depth info at the link above, but that gives you the basic idea.

great thank you !