reducing size of an image in gimp

[Abhishek Dixit]

As per an exam requirement mentioned here
http://ibpsreg.sifyitest.com/cwespljan13/photoupload_static.php
I have to submit two photos who size should
1) Size of the file should be between 20kb-50kb.
2) Size of the file should be between 10kb - 20kb.

I have my scanned photos which have been cropped but the image is of
139 kb size which I have to
reduce in between 20kb - 50 kb image size less than 20kb is not
acceptable (I have tried the system generated an error)

the programm I am using is gimp is there any other program for the same?
If this is possible in gimp then how to do so?
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Abhi