Ok I get the same problem as Mathias and Seafire.
I have changed the BIG_IMAGE_SUBDIR to DIR_FS_CATALOG_IMAGES_BIG as you instructed above. I even tried changing it in general.php as I noticed it was in there too. But no I still get the little thumbs showing instead of the big picture. Please help as this is all that is holding my shop from going live.
Seafire, may I ask is the above all you did to get it working?
Thanks in advance.