Just to add I've deleted all the old images in there... and it's also cleared up another "weird" mystery.
Format card to defaults... take loads of piccys and put card in PC to work with them. What occasionally happened was an "old" piccy from months ago used to show in the "thumbnail" images on the SD card. Just one or two, now and again... how could that be, how could that possibly happen when the cards been formatted. And now I know, a glitch/Windows error ? and it pulled an old image from this hidden folder and displayed that instead. Clicking refresh while displaying the card contents always cleared it.
So remember... you might have some long lost hidden piccys still on your PC.
I'm really pleased to have found this out :)