About a year ago I was looking for some pictures for use in one of my world's fair books, but didn't have everything I needed in my own collection. I saw some very nice shots on a photo sharing site and wrote to the person who had posted them to see if I could use any in the book. He was thrilled to be asked, but it turned out the only copies he had were those small low resolution versions on the web site. After he had them scanned a few years ago he had tossed the original slides and negatives to save space. As a result the images were of too poor a quality to use and I had to continue my search.
If you have scanned any of your own pictures I'll strongly suggest that you resist the temptation to get rid of the originals. Anything could happen to the scanned image, and you might have a need to rescan the original. Another thing to consider is that as technology improves you can often get better scans.
I started scanning pictures on a flatbed scanner, and I think that was at around 300 DPI. That was suitable for web use but not much more. I was then thrilled to get a HP Photosmart S20 scanner, stepping up to 2400 DPI. That seemed fine until I saw the Nikon Coolscan IV with 2900 DPI and Digital ICE. Time to rescan the pictures I had already done!
Then along came the Nikon Coolscan V with 4000 DPI, as well as improved software for color correction... The re-scan saga continues.
It's worth it, though. Here's an older scan done in 2002 with the first Nikon scanner. The subject is the Christus statue at the Mormon Pavilion at the 1964-1965 New York World's Fair. I picked this one as the lighting was especially bad and it was a tough slide to work with.
Here's the 2900 DPI version:
and here's the new 4000 DPI version:
See why I say never throw your originals away?