I having problems with the thumbnail quality on some images where I'd manually defined the thumbnail region. I wanted something a little better than GD's imagecopyresampled, but my hoster doesn't support imagemagick.
In the thumbnail's class I've replaced fastimagecopyresampled with ImageCopyResampleBicubic from the comment thread on imagecopyresampled in the PHP dot net manual. I've also added the unsharp mask function from this page.
Needless to say it is slower and is only a hack, but the results are nicer than GD's default. Maybe we could have an option to use slow/high quality resampling when producing only the thumbnails?