You have to convert the pcx file to SHP(TS) format using XCC Mixer ( a tutorial @ my site says how to do it.
Then in art.ini place Cameo=FILENAME being FILENAME the name of the shp file u got as result.
The cameo can be either put plain in the game folder, or in a Ecachexx.mix file (xx being a number 02-99) or in XCC Mod Creator.
For creating a mix file u will needs XCC Mix Editor ..i think i have made a download available in my site as well...in case i havent....
http://xcc.tiberian.com.
There should be tutorials on how to use it there. I soon will make some tutorials on how to use XCC Mixer and XCC Mix Editor anyway.