How to Upload Header Images in Twenty Ten WordPress Themes
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(Optional) Use the image crop tool on the Crop Header Image page to resize your header image.
The Twenty Ten theme’s default header size is 940 × 198 pixels. Generally, uploading a new header image already cropped in an image-editing program to that exact size is best. However, if your image is larger, you can use the built-in image crop tool to fit the image in the default space after you upload the header.
To resize and crop your larger image, drag one of the eight small handles located at the corners and in the middle of each side of the image. You can also click within the image and move the entire image up or down to get the optimal placement and cropping effect you want.