ImagePad

ImagePad[image, m] pads image on all sides with m background pixels.

ImagePad[image, m, padding] uses the specified padding value or method.

Examples

Add black padding:

img = ExampleData[{"TestImage", "Lena"}];
ImagePad[img, 20]

Add white padding:

ImagePad[img, 20, White]

Please visit the official Wolfram Language Reference for more details.