ImageSubtract

ImageSubtract[image, x] subtracts a constant amount x from each channel value in image.

ImageSubtract[image1, image2] subtracts corresponding pixels.

Examples

Darken an image:

img = ExampleData[{"TestImage", "Lena"}];
ImageSubtract[img, 0.2]

Subtract two images:

ImageSubtract[img, GaussianFilter[img, 5]]

Please visit the official Wolfram Language Reference for more details.