I cannot explain the technical term for this 'indexed' color distortion but I have indeed noticed this same distortion before with other super high-resolution images, particular with high-res images that have been sourced from a scanner.
Right, so the photo preview display's with the evident distortion in both XnView MP and XYplorer's 'preview' as seen here:



Ahhh




So issue must be explainable to the least if not fixable immediately...Or perhaps there is a setting in XnView MP that I have overlooked no/yes?
Here is the same Hi-res photo image seen above in the screenshots for anyone to test
(5.7 MB):
http://www.megaupload.com/?d=KUWEHCR0
Or
http://www.sendspace.com/file/a9frq3
Thank you for reading.

XnView MP 0.39 | windows 7 sp1 x86 | program settings = xnview.ini