I've managed to get a long way there...
This creates my mirror:-
"D:\PhotoResize400.exe" -f1920x1080 -n -m -u -r "-o Generating <QT><DSTPATH><QT><EN>" "-cD:\Pictures_Resized\<PATH><NAME>.JPG" "D:\Pictures"
However:-
1) If a file has an extension of ".jpeg", in the resized version it's changed to ".jpg"
2) Just need to work out how to do the mirror. ie: If a file/folder is removed in the source, how to remove it from the destination?