PCX files are graphic image files that store Paintbrush bitmap graphics.
This raster image format was developed by Zsoft and was one of the original bitmap file formats to be used on the Windows and DOS operating systems.
The PCX file format supports 24-bit color graphics as well as 8-bit grayscale and indexed color images. PCX files are compressed with RLE encoding. A variety of scanning and digital fax applications use the PCX file format to store digital fax copies.
There is a multi-page version of PCX, used by some computer fax and document management programs, with file extension .dcx. A DCX file consists of a header introducing a set of PCX formatted data.
 For more information
- http://en.wikipedia.org/wiki/PCX The actual file format is in this article.