ImageTrim

ImageTrim[image, roi] gives the smallest subimage that includes the specified region of interest roi.

ImageTrim[image, roi, r] adds a margin of size r.

Examples

Trim to bounding box:

img = ExampleData[{"TestImage", "Lena"}];
ImageTrim[img, Rectangle[{100, 100}, {300, 300}]]

With margin:

ImageTrim[img, Disk[{256, 256}, 100], 20]

Please visit the official Wolfram Language Reference for more details.