Especially sending to image editing software with a different paradigm such as the Gimp or Photoshop, a de-noiser like Neatimage or Noise Ninja or possibly filters or scripts.
Ideally, picasa would give the “editor” a temporary file or its own working copy. Once the images is saved again by the external editor, Picasa could enter the revision to its own version tracking so you could “undo” external edits etc.