Use a FileReader to encode your image as a data URL:

jQuery.ajax({...})
.done(function (r) {
  var reader = new FileReader(
  reader.onload = (function(self) {
    return function(e) {
      document.getElementById("img").src = e.target.result;
    }
  })(this);
  reader.readAsDataURL(new Blob([r]));
});

Java 8 has come up with java.util.Base64 package to help encode bytes to Base64 and vice-versa

import java.util.Base64;


byte[] bytes = string.getBytes();

// returns byte[]
Base64.getEncoder().encode(bytes);

// returns String
Base64.getEncoder().encodeToString(bytes);

you should use the .NET methods Convert.ToBase64String and Convert.FromBase64String.

Use the Convert.FromBase64String( ) method. This will give you the binary data back (as a byte array).

To convert binary data to a Base64 string see conversion functions from binary data to a string in vbscript

from http://www.motobit.com/tips/detpg_Base64Encode/

Function Base64EncodeBinary(inData)
  Base64EncodeBinary = Base64Encode(BinaryToString(inData))
End Function

Function Base64Encode(inData)
  'rfc1521
  '2001 Antonin Foller, Motobit Software, http://Motobit.cz
  Const Base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
  Dim cOut, sOut, I

  'For each group of 3 bytes
  For I = 1 To Len(inData) Step 3
    Dim nGroup, pOut, sGroup

    'Create one long from this 3 bytes.
    nGroup = &H10000 * Asc(Mid(inData, I, 1)) + _
      &H100 * MyASC(Mid(inData, I + 1, 1)) + MyASC(Mid(inData, I + 2, 1))

    'Oct splits the long To 8 groups with 3 bits
    nGroup = Oct(nGroup)

    'Add leading zeros
    nGroup = String(8 - Len(nGroup), "0") & nGroup

    'Convert To base64
    pOut = Mid(Base64, CLng("&o" & Mid(nGroup, 1, 2)) + 1, 1) + _
      Mid(Base64, CLng("&o" & Mid(nGroup, 3, 2)) + 1, 1) + _
      Mid(Base64, CLng("&o" & Mid(nGroup, 5, 2)) + 1, 1) + _
      Mid(Base64, CLng("&o" & Mid(nGroup, 7, 2)) + 1, 1)

    'Add the part To OutPut string
    sOut = sOut + pOut

    'Add a new line For Each 76 chars In dest (76*3/4 = 57)
    'If (I + 2) Mod 57 = 0 Then sOut = sOut + vbCrLf
  Next
  Select Case Len(inData) Mod 3
    Case 1: '8 bit final
      sOut = Left(sOut, Len(sOut) - 2) + "=="
    Case 2: '16 bit final
      sOut = Left(sOut, Len(sOut) - 1) + "="
  End Select
  Base64Encode = sOut
End Function

Function MyASC(OneChar)
  If OneChar = "" Then MyASC = 0 Else MyASC = Asc(OneChar)
End Function