HEADERS

You can use a single header for all devices if you want. However, I find it is often a nicer look to use a smaller size for mobile devices. When using a single image, if you have a "feature" that you want visible on all devices (such as a face), try to place that in  the center, as mobiles will cut off the sides to fit the image.

LOGO

Create this at double the size. For our example logo, it is 200px square, but it contrained by css to be only 100px on the website. This gives much better clarity and sharpness for higher dpi devices. Mac users in particular will appreciate this. 