In the "p40esd.bmp" file select the rattlesnake noseart, the "Arizona", and all the green just around those parts. Okay?
Now, USING that selection, BLACK OUT the area you have just selected. Then WHITE OUT all the rest. Then make it an indexed bitmap and save it as "p40esd_a.bmp" and it should work.
Edit: OPTION 2:
Select the area like I said, then fill it in with a bright purple (or any unique color that you will NEVER USE anywhere else on this bitmap). Index the file, save it as normal "p40esd.bmp". Find the index of that bright purple and memorize/write it down.
Create a .txt file called "p40esd_a.txt". Open the file. Type the number that you memorized/wrote down. No hard return. No quotes. No nothing. Just the number, and save/close the file. That should work as well.