How to resize photograph and signature to certain width and height in pixels and file size in KB?

This post is intended for those who are trying to submit or upload images on websites that ask for certain dimension - both in size and pixels. How to resize photograph and signature to certain width and height in pixels and file size in KB? Most online applications for government exams in India asks for scanned photograph and signature in specific pixel size and image size such as -

 Upload Scanned Images of Photograph and Signature:

    The scanned images of photograph and Signature should be in jpg/jpeg format only.
    Size of the photo image must be between 20 KB to 40 KB
    Size of the signature image must be between 20 KB to 40 KB
    Dimension of photograph image should be 3.5 cm(width) x 4.5 cm(Height) only.
    Dimension of signature image should be 3.5 cm(length) x 1.5 cm(Height) only.