You have several options here in terms of CSS:
1) Give the image a % width so that it scales to fit the available space, or set a different width for it on smaller screens;
2) serve up the image as a background image on larger screens but don't include it in the CSS for smaller screens;
3) give the image display: none for smaller screens.
Option 3 should be avoided, though, as the device still downloads the image, so the user loses pays for bandwidth and doesn't even get to see the image! My preference tends to be option 2, but it depends on your needs. If you want the image to display, you can size it to suit, but for images that are just decorations, I would use the background image method and just not serve it up to mobiles.