If it's good enough for Google, it should be safe. It's a real photo editor online that edits photos online. I may never use a local application again. (maybe).
Create an account—it’s free and simple—and upload your photos to your library. The site pulls from all the various sites where you're likely to store them (Flickr, Facebook, Picasa, Photobucket, etc.)
When you finish a photo, use Save & Share to preserve the photo and send it around. Picnik can post the photo on Facebook, Twitter, Flickr or a personal website.