Changing Background color using Color to Alpha - Gimp

(Possible only if background is single color).

1. Select image.
2. Color - Color to Alpha (Convert specified color to transparent)
3. Click on the From color box to bring up next screen.
    Click on color picker tool on the right side of  HTML notation box and drag to pick the background color of the picture. Then click OK.
4. Add new layer (white) and fill new background color.