sys.UnixShellCommand("osascript -e '(path to pictures folder) as string'")
More better (since you get a newline at the end of the above)
string.trimWhiteSpace(sys.unixShellCommand("osascript -e '(path to pictures folder) as string'"))
The list of possible “magic” folders in AppleScript
Get multiple folder paths in one swell foop
osascript -e '{(path to pictures folder) as string, (path to applications folder) as string}'
returns a comma-separated list.