Presently your preference are wrong, when you double click or 'open' a posted image's thumbnail, what's then displayed is not scaled to the screen and can require scrolling in both directions to inspect it.
I think that 'uploaded' images should be automatically scaled to fit the forum's page size..That way large ones like the one I posted will not occupy as much server space..That's just a thought, encouraging posters to host elsewhere (if at all possible) would be the better option.
The image here is what the mod will do..