Today Google has expanded the collaborative albums feature of Picasa Web Albums to support collaborative editing using Picnik. Now you can invite friends and family to contribute to an album either by uploading photos of their own or by editing any existing photos in that album.
Your original always stays safe and sound as your collaborators make edits to add funny speech bubbles, give you pointers on editing techniques, or pitch in to help you touch up your hundreds of vacation photos. If they edit one of your photos in Picnik and save the changes, it will save back to your album as a new image, and their name will show in the sidebar to give them credit for the new version.
To enable collaborative editing, simply share an album with your friends, and be sure to click on the checkbox to let the people you share with contribute to your album. Soon you will start seeing all the cool new images they create from your photos.