Here's a solution to your problem. Here's an easy way to combine the 144 small images into one big one...
Let me start by saying that I'm operating under the assumption that you're running Win98 or later. If you're running something else, you'll need to modify this a bit.
Go download and install ImageMagick (
http://www.imagemagick.org/script/index.php)
Drop this batch file into the same folder as the 144 PCX files and run it (
http://users.adelphia.net/~1010101/merge.bat). If you're worried about running a batch someone else made, feel free to crack it open in a text editor and have a peek at what it does. Nothing fancy or evil... Just uses a piece of ImageMagick to do the job at hand.
You can then delete the 12 "rowXX.pcx" files, where XX equals 1 to 12. I'd have included that in the batch, but maybe you'll want the rows for some reason.
The finished result will be a file called map.pcx which SHOULD be what you're looking for. Note that I've not tried it myself yet, so I can't attest to the final size, but expect it to be beefy.