Can you recommend how I can create a header in Photoshop CS6 which I can scale to
my choice of container size prior to uploading, but ( like your own GP logo) opens as a high resolution image.
GP uses an SVG image that can be exported from Illustrator ( or other vector application ).
You will need to install the SVG Support or Safe SVG plugin to be able to upload SVGs, then you just need to make sure you set the Logo size in the Customizer.
No overhead they simply allow the SVG mime type to be uploaded to the media library.
SVGs can include code, which means they could include malicious code. This is not a problem if you have a trusted source or you create the SVGs yourself.
SVGs require a vector editing application like Adobe Illustrator – there are free tools like Inkscape but no where near the power of Illustrator. I personally use Affinity Designer which when i last checked is $50 lifetime license – unbelievable value as its on par with Illustrator.