[kabing]

A friend of mine took a picture of my son with her digital camera. When she downloaded it to her (Windoze) computer, the file is a collage of three different pictures. The top half is from the photo of my son; two thirds of the bottom half is part of a picture she took earlier this spring of an Easter Egg hunt, and the last 1/3 of the bottom half part of yet another picture (probably from the same event). The top half is correctly exposed; the bottom two pictures are dark with a heavy blue cast. She says the bottom two weren't even on the memory card any more (obvious they stuck around somehow). It appears the same way in iPhoto.
The really odd thing is that the thumnail that Windows uses for the icon of the file and that Mac OSX uses for the "Preview" in the Finder's "Get Info" command show only one picture-the complete picture of my son.
Is there any way to salvage the correct picture from the thumbnail?
If she still has it on her camera, is it worth trying to download it again?

[Nobody]

You may wish to try something that recovers the files, such as imagerescue.
Generally speaking, it is a good idea to reformat the memory card in the camera before starting to use it the second time.

[dcpics]

The thumbnail is correct because most digital cameras will write a separate thumbnail file at the time of exposure.
The corruption is coming from where on the disk the full image is stored. As said, it is best to reformat the card each time.
The thing is that even a reformat does not erase old images. It simply resets the directory structure saying that it is okay to overwrite old files. They remain on the disk until they are overwritten, but will not appear unless you use file recovery software because they no longer remain in the directory.
This sounds like it could be a directory corruption with it not being sure where the full file is located. Sounds like you are getting part of the correct file and then only one color rendering of the old images.
Unfortunately, if the corruption came at the time the image was written so the full file did not get recorded to the disk then recovery may not be possible.