I see the problem.
Photon adds multiple srcs to your images to show at different screen sizes – this is great because larger images aren’t loaded on mobile for example, smaller images are.
However, this will conflict with the resizer in Page Header, because Photon kind of forces the specific image sizes that it wants, and ignores any external input.
I’ll see if there’s anything I can do for a future update, but I wouldn’t really want to remove the Photon functionality.
Your best bet at this point if you want to use Photon is to resize the image in the Media Library: https://en.support.wordpress.com/images/image-editing/
That way you get the best of both.
Hope this helps 🙂