Finally i had figured it out with the help of my sir.
1)First request the encoded string image by
$encodedImage = $_REQUEST;
2) Then create an image name with ur own requirement
$imagename = $monitorId.'.jpg';
Note that the extension of the image should be the same as the image that is posted from the .net application.
3) Open a file with modes w+ or wb
$ifp = fopen(ROOT."/images/monitoring/".$imagename , "w+");
4) then replace all the spaces in the encoded string with + symbol
$encodedImage = str_replace(' ','+',$encodedImage);
5) Then write the encoded image string to the file.
while writing the encodedImage should be decoded using base64_decode ()
fwrite( $ifp, base64_decode($encodedImage));
6) fclose($ifp);
That is all... now go and look the /images/monitorig folder ... there will be ur image ready to smile for u....