Didier Stevens at SANS ISC analyzed TerminalFix PNGs that carried a DLL payload inside otherwise valid image files. The files had normal headers, no appended junk, and decompressed cleanly, yet their IDAT data reconstructed a Windows executable.
The trick is simple and hard to spot: the payload sits in the PNG pixel data itself, spread across the compressed image stream, so a scanner looking only for appended malware or odd wrappers sees a legitimate picture. That means the executable only appears after decompression and reconstruction of the image lines.
For email, chat, and web filters that allow images through by file type or by checking only end-of-file anomalies, this leaves a blind spot. The exposure is in the content of the image stream, not in the filename or the tail of the file, so the same control surface can miss a payload that still looks like a normal PNG.