Andy said,
"BTW, I saved the header image at 72 resolution, and it uploaded almost instantaneously as the header background image."
That's a great point, Andy! Remember that for online viewing, 72dpi is all that's required. Banners, avatars, icons, and such are for viewing ONLY -- not to be printed -- so only need to be 72 dpi, in most cases. This is also true for images posted to your blog and website. Any image that's to be viewed only, and doesn't need to be printable, requires only 72 dpi.
Keeping background and 'show only' images low-res will allow your web pages to load faster, making for a better UX (user experience) for visitors!