Follow this step to make transparent image.
1. Open The GIMP and create a new file or open an existing file you wish to create transparency on.
2. Select the "Selects regions by color" tool or press Ctrl + O. Once selected click the portion of the image you want transparent. For example, clicking on the white background of an image. If you wish to not have borders around the non-transparent portions of the image you will need to increase the Threshold by dragging the slider to the right.
3. Once the area you wish to make transparent is selected Click Layer, Transparency, and then select Add Alpha Channel.
4. Click Edit and then select Clear or press Ctrl + K. The selected portion should turn now appear as a checkered background indicating it is transparent.
5. After you've got the transparent background you want click Image, Mode, and then Indexed. In the Indexed Color conversion box change any settings you wish or simply click Ok.
6. Finally, once the above has been completed save the image as a .gif image for example mypicture.gif.