Just started using this plugin and already I'm floored at what it does. Installed and is operating flawlessly so far.
Makes uploading, cropping, resizing, rotating and watermarking images as simple as clicking a button. I don't think it could get any easier or more powerful than this - for free.
Perfect for situations where your client needs to upload images, but doesn't know how to use photoshop for image editing - this plugin does it for them. (Not sure about image upload sizes...)