Last I checked, most browser don't support bitmap files. Choose some other format that is widely supported, like .gif, .jpg, or .png.
You may also be running into issues with the lack of "web-safe" colors, and the fact that every computer, monitor, display card, browser, and operating system will change the way an image is viewed by some small amount. A high-resolution image with lots of colors in it may look great to you, but look crappy to people visiting your site.
I strongly suggest finding some of Robin Williams' web design books, which are marvelous and probably available through your local library (Web Design Workshop is probably the best for your purposes).
Read up on the "web safe palette" a bit and some of what I've said might make more sense :-)
Good luck!
