Im having a major headache changing the header images.

ive downloaded various pictures and cropped them to various sizes (including the sizes that come with the theme)

when displayed in the site, it either cuts the picture in half and doesnt stretch far enough across (only displaying the top half of the pic), or streches too much and cuts off each side of the image (showing the full height of the image but not the width)

is there a simple size etc, that i have to crop to, to display the image correctly please?

many thanks

There are two position and size related settings for the header images at A. Theme Options -> Header Images:

Header Image: Height

and

Header Image: Alignment

Also, the header images must have no spaces or special characters in their file name, and the file name must end on .jpg, .gif or .png