Often I am facing one and the same issue - deleting backup folder of Picasa. There are usually lots of rotated images or movies. So usually this is the first place to search for free space.

Here is one very short way to delete these subdirectories:

cd ~/Pictures
find . -iname '.picasaoriginals' -exec rm -rf '{}' \;

This will remove all subdirectories from your “Pictures” folder that have name .picasaoriginals.