Automatic crop of a picture from a post to use these as header

If a post has his own picture, I can use this in the header also. But what happen if these pictures has totaly other dimensions as the header picture? Then you have these big picture in the header....so this makes no sense. My question: Is there a automatic way to crop this picture into the same dimension as the standard header picture? I don't like to upload 2 versions of a picture (one for the post, one for the header...)