How do you copy and paste a picture from gimp as a background?

i use paint tool sai for my linework and coloring my character. i want to create a background for my character in gimp and then put it as my background for my character. the thing is i take a picture of my actual drawing of my character then trace it and color it using paint tool sai, so i open the photo and trace and color. Im not sure at all what to do?