The Wireless Bitmap format supports only two colors, and making images look good can be tricky. However, your favourite image editor might have a function for decreasing the number of colors, and it might even have a method of doing this called Floyd-Steinberg. One such program is Jasc Paint Shop Pro. Paint Shop Pro is inexpensive compared to others, and there's even a WBMP plugin available which allows you to save WBMPs directly. You can download a 30-day evaluation copy of Paint Shop Pro at the link above.
 
Here's some examples on the quality of the Floyd-Steinberg method.
 
If you have access to a server side scripting language such as PHP, the same deck can be written much simpler like this: