I use "Digital Photo Editing for Seniors" by Addo Stuur and it's excellent. It comes with a book, in large print, which has actual lessons it it for you to follow. It is a book with 400 pages, so not just a manual, and the CD program is in the book. I paid about $35 for it a couple of years ago.
Picasa only has basic tools for picture adjustments, such as colour setting adjustments, not the more complicated tools e.g. those for modifying areas within a picture, cloning, and so on. GIMP does have those, so does Paint.Net (which I prefer to GIMP) as well as Photoshop Elements.