Do it with the GIMP → Basic Areal Transformations → Scaling an Image

5.1. Scaling an Image

In order to scale an image use the <Image> -> Image -> "Scale Image..." menu option.

Demo

Open tiger_sitting.jpg
Select <Image> -> Image -> "Scale Image..."
Increase it by 3 in each dimension. (you can lock them together)

Written by Shlomi Fish