ImageReflect

ImageReflect[image] reverses image by top-bottom mirror reflection.

ImageReflect[image, side] reflects so the specified side goes to the opposite side.

Examples

Vertical flip:

img = ExampleData[{"TestImage", "Lena"}];
ImageReflect[img]

Horizontal flip:

ImageReflect[img, Left]

Please visit the official Wolfram Language Reference for more details.