Ooh this is handy. Just posting this here so I can go and find it again.
http://help.squarespace.com/customer/portal/articles/770320-how-do-i-modify-or-create-images-for-my-site-?t=103959
Some free and paid options for editing images to be found in that list, which is helpful for those who don't have CS or don't want it, or are deciding that they do need it.