Ask YC: What's the best way to resize images for web applications?

So our web app is starting to deal a lot with images of various sizes... per user. So lots of images. We've already ran into FS issues and whatnot, but now we're trying to decide the best way to 'code' resizing images functionality.

No, we don't want some Java applet. Yes, we'd like to do this all in the background. Ie: as soon as a user uploads a photo, a script would resize it, make copies of diff sizes, etc...

What languages have good image support? What about compression?

I heard Facebook is doing / did research on this to retain image quality when they do their resizes... any thoughts?

THANKS!