Automating thumbnail generation on high traffic

I'm building a Zend Framework application which uses a lot of photos in different sizes (thumbnails).

I plan to generate the thumbnails asynchronously:

1. user uploads the photo
2. the server generates thumbnai开发者_如何学运维ls in all sizes (eg. using gearman worker)
3. all the images are uploaded to the Amazon S3.

Is there any good tool for generating thumbnails (resizing, cropping, sharpening) on the server, or PHP + GD will be fine?

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I would recomend phpthumb. I use it, and I have no complains so far.