With GIMP you do the following:
1) Open your picture in GIMP. On the file menu at the top, choose [Select -> All] then [Edit -> copy]. Close the picture. Don't save. Now in your GIMP title bar menu choose [ File -> Acquire -> Paste as New]. We'll be working with this copy (the advantage being that you won't screw up the original).
2) Now use the Select by Color tool to take all of the ugly pink and green out of the picture. The SbC tool selects the color: [Left Click ->Edit->Cut]takes it completely out. Remember, you can always do [Edit->Undo] to fix a mistake, or several in a row.
3) Now choose [Select -> All] then [Edit -> copy]. Then you want to index it: [Image -> Mode -> Indexed ] A little dialog comes up; you want to pick [Generate Optimum pallete] and set the maximum colors to 255.
4) Open [Dialogs ->Colormap] and a nicely indexed pallete should come up in a box. If it is just a bunch of grey squares go back to step 3. Now here's something I discovered: click on the last color box. If there are less than 255 colors you have to click on the last color and then the little "+" sign to add duplicates of the last color until you get to 255. Otherwise the program will add those extra boxes later and screw up your palette.
5) Then edit the colors in this order: Left-Click on the second to last color and choose [Edit Color]. Where it says [HTML notation] make it say 00ff00. Click on OK, and then repeat the process with the last color: its [HTML notation] will be ff00ff. Click on that last color (pink) to choose it before you close the Colormap box.
6) now go back to your original picture. Going back to the SbC Tool, click on an empty area to select it, then [Left Click -> Edit -> Fill with FG Color.] (The FG color should be that special pink. If it's not, go back to the Colormap and pick it) That should fill all the empty areas pink.
7) Remember the Copy we made at the beginning of step 3? Go [Edit ->Paste ].
8) Now go [File ->SaveAs] and put in the file name, being sure that it's converted to a pcx. I just type "filename.pcx" in and it converts it for me. That should do it.
Now you'll tell me you don't even use GIMP, and I've just wasted a bunch of space. Well, maybe someone can use this.