I have read these forums and it 50% of the time people solve this by changing their permissions on the upload directory to 7-5-5.
The other 50% of the time, people solve this in the dashboard by going to SETTINGS > MEDIA and in the "Uploading Files" section there is a field that says "Store uploads in this folder".
Make sure the field says this: wp-content/uploads with no quotes, no leading slashes or anything else. I know other people reported problems by having a leading slash.
We are having similar issues even after trying the above, but those work for 99% of other people.
So,,, we looked opn our server using FTP, and saw the images were not in the wp-content/uploads directory! We then FTP'ed the 3 images directly to the wp-content/uploads directory on our server, and that got the images to show up in our blog! Now the thumnails are showing up in the MEDIA > LIBRARY of the Dashboard.