First off, I seriously doubt that size-wise, you'll see much improvement by zipping a .BMP file. They just don't compress well at all.

I have found that .PNG files are the best type to use when size matters and the compression of .JPG files is unacceptable. .PNG files can be quite small but retain very good clarity.

I hate to admit it but I haven't used the built-in image viewers in Windows for a very long time. So, I don't know if they can show .PNG files. I do know all other freebies and paid viewers I have used can...