Press 'r' for randomness.
Press 'h' to toggle interface.
Regenerate

View on a bigger screen for more control.

Brightness

Slice the image into square pieces and randomly manipulate their brightness.

Logic: The image is divided according to the Grid Size setting. Next, random cells are selected and their brightness is randomly changed (from -100% to +100% in 20% increments). The Percentage parameter controls the chance of the cell to be manipulated. Upload your own images to experiment.