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In Google Chrome, hit F12 on the page to bring up developer tools. Click on the "Elements" tab. You can then browse through the html until it highlights the image. You'll see the exact size of the image in the styles section.
If you're looking for the max width for an image in the center column that you can get without creating scroll bars when the page is minimized, I believe it's 412px. I'd recommend uploading the image and then setting the width to 100% and the height auto so that it changes size with the size of the browser (make sure to center the image)
I used 1350x150 pixels as the header size in Photoshop (best guess). Seems to work ok.