The DiagnosticJson plugin can return diagnostic information in place of the requested image.
Sample requests:
/red-leaf.jpg?maxwidth=300/red-leaf.jpg?maxwidth=300&rotate=90/red-leaf.jpg?maxwidth=300&rotate=30/red-leaf.jpg?crop=10,20,100,200/red-leaf.jpg?sRotate=90&crop=10,20,100,200