Actually CombineZ is a bit easier than I thought at first, but the documentation is a bit overwhelming.
To make it work, Click 'File' then highlight the files (referred to as 'frames' in CombineZ) that you want to stack. Then under the 'Special' Menu, click 'Do Combine' - that's it - you get a BMP file as the final product. I suspect there are a zillion options available...
