You don't need to resize photos that are already online unless you want to make them a certain size on the screen. You can just change the size attribute tags in the image code. Follow my tech link below and look for the page about adding images.
Just keep one thing in mind, adding an image to the HEADER will put that image on EVERY PAGE, including the posting page. If that's not what you want, then put it at the top of this section:
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