How do you put a white boarder around a cartoon image in Gim

[FromtheOldManor]

I saw this picture on DeviantArt a couple of months ago where this guy drew Hiccup and Toothless from How to Train Your Dragon on photoshop or something. He first drew the two characters and colored them in; then colored in a background. After he was done with that he did this effect where he added a white border around Hiccup and Toothless to separate them from the background and make them look 3 Dimensional. I was wondering how you'd do that effect on Gimp or photoshop.