Ha! have worked it out. For anyone else having this problem:
Casio cameras are unique (apparently) in their ability to create a web album on the fly. Therefore you have a pic directory (casio100) and album directory (album). In the album dir are lots of 320x240 images - copies of all of the ones on there - deleting these will probably fix the problem im guessing as it means there will only be one copy of each image. Hopefully I can turn off the album as I never use it!