A question about thumbnails

I’m having a problem with thumbnails not showing correctly and would appreciate some advice.

I set the thumbnail option in Media Setting to 66×66 and when I uploaded images via Media – Library – Add New and checked for the images in my ftp program I discovered that the thumbnails coming through were 66×63 and I don’t understand why this should be so and if that is what is stopping my images from appearing.

Many thanks for any help offered