CgBI

Overview
CgBI is the informal name of a PNG-like image file format developed by Apple. It may also be called iOS optimized PNG.

It is not compatible with PNG. Standard PNG decoders that don't support it will fail gracefully, due to an unknown "critical chunk".

Identification
Files begin with the standard 8-byte PNG signature. A private "CgBI" chunk appears before the "IHDR" chunk.