How do I convert mcrypt_encrypt from PHP into .NET code? - php

Question

I'm looking to convert this function from PHP to VB. Could anyone help me understand why the output is not correct?
//get img
$img = file_get_contents('https://www.random.org/analysis/randbitmap-rdo-section.png');
//pad
$blocksize = 16;
$pad = $blocksize - (strlen($img) % $blocksize);
$paddedimg = $img . str_repeat(chr($pad), $pad);
//encrypt
$data = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, 'abcdefghijklmnop', $paddedimg, MCRYPT_MODE_ECB);
//output
echo base64_encode($data);
The above outputs;
wYkugZmmsCHNwFOr7sAgk6yUU2I+2rOtBmXAIcPleue+dT4zIvkSA6UYmW6tickeoIbyCylxrb5n ...
[rest truncated but goes on and total is 29484 characters]
Here's what I have in VB;
Public Sub GetBase64EncryptedImg()
Dim Img As Byte() = Nothing
Using wc As New WebClient
Img = wc.DownloadData("https://www.random.org/analysis/randbitmap-rdo-section.png")
End Using
MsgBox(EncryptECB(Img))
End Sub
Public Function EncryptECB(Data As Byte()) As String
Dim Key As Byte() = Encoding.UTF8.GetBytes("abcdefghijklmnop")
Dim Encrypted As Byte() = Nothing
Using rj As New RijndaelManaged()
Try
rj.Padding = PaddingMode.PKCS7
rj.Mode = CipherMode.ECB
rj.KeySize = 128
rj.BlockSize = 128
rj.Key = Key
Dim ms As New MemoryStream()
Using cs As New CryptoStream(ms, rj.CreateEncryptor(rj.Key, rj.IV), CryptoStreamMode.Write)
Using sr As New StreamWriter(cs)
sr.Write(Data)
End Using
Encrypted = ms.ToArray()
End Using
Finally
rj.Clear()
End Try
End Using
Return Convert.ToBase64String(Encrypted)
End Function
The VB outputs;
pVngUaXYYv1tJSPzjDuwKw==
That's it, nowhere close to the right number of characters. Where am I going wrong here?
EDIT: Re being marked as duplicate, that question/answer gives back a base64 encoded string. mcrypt_encrypt doesn't, so I'm not sure how to apply the code in that answer.
EDIT2: That duplicate gives uses an IV value whereas the code above doesn't. Would that not differentiate the two to deserve a unique answer? Or is it as simple as removing the line setting the IV property of the RijndaelManaged object?
EDIT3: Completely reworked the question