Chapter 6, you'll learn to apply the photographic-style filters on the Effects tab to your pictures.No matter which tab you're on, the bottom of the panel has two buttons: Undo and Redo. Click them to reverse the effect of the most recent edit (Undo) or reapply the most recent effect you've removed with Undo (Redo). The button labels change to reflect the specific edit/command you can Undo or Redo. Because Picasa tracks all edits for each image, you can click Undo repeatedly to remove a series of editsall the way back to the original image, if you like.
Undo and Redo buttons
The edit sequence is stored with the picture. If you open it at a later date in Picasa, you can still use the Undo button to step back through your edits.