Changing Background using Select by Color Tool-Gimp

1. Open new image.

2. Right click image layer - select Alpha Channel.

3. Click Select by Color Tool.

4. Check Antialiasing, Select transparent areas.

    NB: Higher Threshold setting will result in more similar colors to the one you clicked on will be   selected.

5. Now pick the color you wish to delete.

6. Edit clear. (For areas not selected, you can repeat this process or use Fuzzy Tool to select.

7. Now you can add new background by adding new layer.

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.