About the adjustments
Brightness lightens or darkens the image uniformly. Contrast increases or decreases the difference between light and dark areas. Saturation boosts or reduces color intensity - at -100 the image becomes fully grayscale. Sharpness applies an unsharp mask which enhances edge definition, useful for photos that look slightly soft after resizing.
The live preview is canvas-based and runs entirely in your browser. The actual export is processed server-side with ImageMagick for maximum quality and full transparency support.