After more consideration, probably this setting "Who can access your original image files" set to "Anyone" should be optional.
I just realised something about flickr. It seems around the end-May 2010, flickr changed their image creation system to include the 'Large' size for originals of 1024 px or bigger (previously I guess uploads of 1280 px or bigger triggered the generation of 'Large', as indicated above in my original post). So some older images may not have a 'Large' size (1024 px) so the next size down is Medium when 'original' of 1024 or larger is unavailable due to this option being other than 'anyone'.
So if all your uploads have a 'Large' version and you are happy with 'Large' being the max available size (in place of the original not being available due to the above flickr setting not being set to 'anyone') then you don't need to set that option.
The downside to enabling 'anyone', is you then allow anyone to come to flickr to download the original image. It seems most newer images always have Large (1024px) versions available, so this may suffice and you can safety not set that option.
It would be great if flickr somehow allows originals to be available via this gallery, but disabled downloading. I guess it's not possible with the current method.