Gallery preview folder takes about 1GB, how to delete it?
Do you really want to do that? That might lead to you no more being able to see your images in the gallery. But if you want to try:


open a file manager of your choice and navigate to the DCIM folder
delete the .thumbnails directory
create a file named .thumbnails
open the file's properties, and set it "read only"


Now, as there cannot be two objects with the identical name, the gallery cannot re-create the folder. As the existing object is a file, it cannot put its thumbnails in there, either.