URI standard: https://www.ietf.org/rfc/rfc2396.txt (https://www.ietf.org/rfc/rfc2396.txt)
PNG standard: http://www.w3.org/TR/REC-png (http://www.w3.org/TR/REC-png)
MD5 hash algorithm: https://www.ietf.org/rfc/rfc1321.txt (https://www.ietf.org/rfc/rfc1321.txt)
