I received a contact through the contact form a little while ago from a Carvewright Designer user trying to import the tiff file. Unfortunately the email address was invalid so I’m going to post the response here and hope he sees it.
According to this site, the carvewright designer imports stl models. There is an stl file in the zip and this is the method I'd recommend for loading this model. If for some reason you need to use a grayscale bitmap then the tiff is a much higher resolution bitmap than a bmp file, that's why I use it. If bmp is your only choice then you can load the tiff up in microsoft paint or any other image editor like gimp and just save as bmp, then load it into designer. If you do stick with tiff or bmp you will need to increase the z height of the model since bitmap formats flatten models. You will also need to re-smooth it. I include the tiff as a last resort if you can't import any of the others. If you can use the stl you will have much better results. Good luck.