The problem you describe is most often the result of corruption of the file by the memory card. It can be caused by any link in the chain between the in camera processor and the screen you view it on. A bad cable or bad connection to your computer, a bad hard drive, or other internal problems that corrupt data as it is transferred inside your camera or computer can also cause the same problem.
Keep in mind that the image displayed on your camera's LCD screen is often a reduced size preview image generated by the camera and stored with the image rather than the actual full size image itself. The preview thumbnails can be fine at the same time the full sized image is corrupted.
To determine the source of the problem there are several things you can try:
- Reformat the card using the camera. Sometimes reformatting a corrupted card will solve the problem.
- Try a different card, preferably one not from the same manufacturer as the original card and certainly not from the same production batch. If you still have the problem it probably isn't the card.
- Try transferring the images to a different computer.
- Try the original card in a friend's camera. Don't forget to format it in the new camera first.