Generated thumbnail has black top border

I use the Media Gallery for all my images and specify a fixed size for thumbnail images. It’s directly proportional to the full size images, which are all the same size.

However, for some irksome reason, the thumbnail images are generated with a black, 1px border on the top of them – it stands out like a sore thumb on light thumbnails.

Any idea how to stop this happening, so I can delete all my images and reupload them?

Thanks!