In FTK, there may be a situation where a carved image displays what appears to be a blank or empty thumbnail, like below:


However, when selected, you can see the image, as shown below:

In the above example, the image is carved, and the resolution of the image is 18977 x 23 dpi.
(Based on the image 353 x 371 pixels when converted to dpi, the result is an image that would be about .02 inches wide and 16.1 inches tall.  When converted to a thumbnail, there is nothing to see.)

To see the actual image that the thumbnail was derived from:

1. Select the file in the graphics tab, 
2. Right click on the image in the viewer
3. Click: Size > Use Image Size.