Also another thing Picasa has developed in recent versions is that some edits you make are not applied directly to the photo but are instead stored in a seperate file. That means when you look at them with Picasa, the edits are visible because it reads that little edit list file and applies it, but, looking at those same photos with another program or just as thumbnails in windows explorer, the edits will not show since any other program does not read in that seperate edit list.
You can take actions to save the photo with the edits applied but right now there are not very good options for controlling the quality of the re-saved photo with the edits appied, so I wouldn't recommend it, but I should also note I haven't gotten around to trying 3.5 yet so that may have been improved (abilty to save as .bmp format would have been an easy solution).
I think there's a good chance the My Pictures photos and those that Picasa sees are really the same ones despite the edits so I wouldn't recommend deleting any you want. You might try moving (not with Picasa) a subfolder of photos to an unwatched (by Picasa) area of your drive(s) and see if they stay in Picasa or disappear. You can also right click on any photo in any folder set in Picasa and choose locate on disk...the window that pops up will reveal the location of those files. If its in My Pictures, chances are very good that they are one and the same location.