Header resizing question twenty eleven

Hello. I know the default header size for twenty eleven is 1000×288. I have tons of images being used successfully and randomly that wordpress cropped and resized for me. But I was trying to use an image that wouldn’t fit. So i went into Photoshop and cropped and resized the image to EXACTLY 1000×288. That new image was still way out of proportion for the header. I searched around for an answer with no luck. Any suggestions? Resolution have to be something specific. Thanks or any insight or suggestions.