I used to do a pretty frequent task to find any image in my media directory, which has atime longer ago than a week ago, which hasn't already been crunched in some previous round, and crunch the hell out of it with either mogrify -quality or pngquant. All of these softwares like to keep full-quality copies of a basically infinite number of images which there's about a 99.9% chance will never be needed again, and you can crunch them down to a tiny fraction of their original size, and they still honestly look more or less fine.