slip-0039: use nicer images from point software

slip-0039.md

@@ -60,7 +60,7 @@ For checksum we use lower 16-bits of CRC32 algorithm. We don't use CRC16, becaus
 
 When enough M secrets are provided the master secret is reconstructed. To allow an additional protection of the final seed using a passphrase we suggest a following scenario. The passphrase is hashed using PBKDF2, which servers as a key to an AES encryption. The result of this encryption is the final seed to be used. If no passphrase is provided the passphrase is set to an empty string.
 
-![passphrase](slip-0039/passphrase.png)
+![passphrase](slip-0039/secret_sharing_3.png)
 
 This mechanism is intentionally symmetric to allow reconstructing the master secret from the seed in case the passphrase is known.
 
@@ -82,3 +82,7 @@ Wordlist mandated by this SLIP is ![available here](slip-0039/wordlist.txt). Sev
 * wordlist contains only common English words
 * no word is shorter than 4 letters and longer than 8 letters
 * all words have unique 4-letter prefix
+
+## References
+
+* [Secret Sharing Step by Step by Point Software](http://www.pointsoftware.ch/en/secret-sharing-step-by-step/)