Change The Default Header Size.

I have the Simplish Layout for my blog and I want to change the header size but it seems to set to : exactly 900 × 62 pixels as-is.

I have altered it to the size I want, http://www.thewhitezebra.com, but when I upload the image is only lets be crop a portion of the image and not the whole image giving me a duplicated image look ( if that makes any sense)

Can any one help?

Can i remove the default cropping function on my wordpress.

Thanks

twz