The PCX file found in the unitpics subdirectories is essentially a simple 96x96 pixel image of your unit, typically placed on a nifty-looking background. This is the picture of the unit that is displayed when you hit the F1 key during a game to bring up the unit's information.
To create your PCX file, use any of a number of graphical editing tools, such as Paint Shop Pro, Windows Paint, etc. When you are finished creating your picture (whether it be a BMP, GIF, or PCX file), save the file as a PCX file. The image file should be placed in the unitpics subdirectory, and given the short name of your unit as the filename "shortname.pcx".
A small pack of background images is availabe for you to download and use from Fileuniverse here.
Rather than embarassing yourself with a free-hand stick-figure drawing, you could use
the modified Unit Viewer that comes with Kinboat's
3DOBuilder application.
Using the same feature that you use to create the unit's GAF image, 3DOBuilder
will create a BMP file of the appropriate size for you.
Most quality unit makers however, go
even further and create their unit pics using an application like
3ds max or
Lightwave.
Finally, you will need a tool like Photoshop to convert this final image to PCX format.
It must be in the 8-bit color format to be used by TA.
For people wanting an in-depth specification of the actual file format, you can view this page: