Original photo quality when saving
Picasa is designed to protect your original photo files as you work with your files. When you decide to save your edits, Picasa creates a new JPEG copy of the photo with all of your edits applied, leaving the original photo unaltered on your hard drive.
JPEG is a lossy file type, which means that each time you save the photo, you'll lose some information from that photo file. This is the nature of the JPEG file type, and not specific to Picasa. This information loss occurs because the creation of the new JPEG requires a certain amount of JPEG compression. The more times you save a given photo, the more photo information that will be lost.
You can control the JPEG compression level of your edited files by exporting your photos, but a JPEG file will always be recompressed when saved.