>>1
Some image formats (like JPEG) allow you to store a thumbnail as well as the normal image. As a result, programs that use the embedded thumbnails can be fooled if the thumbnail is not an accurate representation.
In the case of GIF, you can make an animated image containing two frames. The first is what the thumbnail will show, but if you set the frame delay to the smallest possible value, what people will see is the second image, not the first.