Fast Image Library for Thumbnailing?

[Kung Foo Ham[p]?ster]

Can anyone recommend an image library that can resize large images 
(~ 2048x1600) down to thumbnails incredibly fast.

The quality can be low(33%) but I would prefer that it didn't look horrible.

I have been toying with ImageMagick, but I was wondering what other people 
would recommend.

What do you have to say about imlib2, gd2, libart, libjpeg? What would be good 
for this task?

Thanks!

-Adam