Picturesque meant - he decided after careful observation of the scenerey that inspired Twoflower to use the word - that the landscape was horribly precipitous. Quaint, when used to describe the occasional village through which they passed, meant fever-ridden and tumbledown.
Twoflower was a tourist, the first ever seen on the discworld. Tourist, Rincewind had decided, mean 'idiot'.
Source: The Color of Magic, Pages: 101
Contributed by: abarrach