hi dave,
ok, i open a picture, i had the guides 'on' set to 3rd's so my pictures are quite balanced, in order to move/adjust the picture slightly this way or that, i use the Crt T key (transform), i adjust the picture then either double click or tick the arrow.
OK, so now i want a bigger canvas size, open the canvas menu, type in the 2cm black(usually) size increase, but the black only covers 3 sides of the picture, the side i had adjusted/moved doesn't get any extra size at all, whats going on..