What is the difference between "image/png" and "image/x-png"?

Question

What is the difference between "image/png" and "image/x-png"?

Answer 1

The x- prefix is given to non-standard MIME types (i. e. not registered with IANA). So I assume that image/x-png would have been PNG before the MIME type was standardized.

6.3. New Content-Transfer-Encodings

Implementors may, if necessary, define private Content-Transfer-Encoding values, but must use an x-token, which is a name prefixed by “X-”, to indicate its non-standard status, e. g., “Content-Transfer-Encoding: x-my-new-encoding”. Additional standardized Content-Transfer-Encoding values must be specified by a standards-track RFC. The requirements such specifications must meet are given in RFC 2048. As such, all content-transfer-encoding namespace except that beginning with “X-” is explicitly reserved to the IETF for future use.

—RFC 2045 — Multipurpose Internet Mail Extensions, Section 6.3

This is also documented in the PNG specification. See FalseVinylShrub's answer.