I've added libvips as a third option for thumbnail generation. It's better than ImageMagick in that it is around 4x faster, and uses 1/10th the memory. It is particularly good if your webserver has more than one core available as it will split the work across multiple threads, whereas ImageMagick won't. Like ImageMagick, it is colour space aware (GD isn't).

In terms of speed and quality, the options are now (best to worst):

• libvips
• ImageMagick
• GD (default)

GD is best avoided, and is the default only by virtue of being widely available on most web hosts, whereas ImageMagick is sometimes available, usually with some configuration, and libvips usually isn't a PHP thing.

To enable it, install libvips-tools on your system and rename ResizeImage-Vips.php to ResizeImage.php (and backup or remove the existing one). Refer to the files's docblock for instructions, and to configure compression and sharpening.