ckfinder browser displaying filetype icon instead of image t

Hi,
ckfinder (php) seems to be working correctly, in that I can edit and upload to directories etc, but the folder browser displays filetype icons instead of thumbs of the files themselves. I have had a look in documentation etc to try to find a solution and it may be that the php gd extension isn't installed, but I wanted to ask if there are any other reasons why this might not be working?

Tony