Ok, this is where you've got some choices.
First and easiest... Save the BMP file with the 24 bit selection. Now close your layered file, and open up the BMP file you just saved. Click 'Image : Mode : Indexed Color'. This should bring up some options in a dialog. For the palette you want one called 'adaptive', and then set dither to 'diffusion' and a hgh percentage, 100% is fine. Save the file and it should work in IL2 now.
The longer but visually improved method is to use BRIGHT, an external application which does the conversion to a higher standard. There is a whole separate article covering this process, here:
http://www.simmerspaintshop.com/foru...ng-bright.html
And if you want to know some more about what's going on with this conversion process in the first place, this article will explain some of the details:
http://www.simmerspaintshop.com/foru...e-artists.html
Lots of people get stuck on this for the first time or two, then it's easy as pie. Feel free to post if you get stuck!